azure cli: verifying we're authenticated as a User

Unfortunately when authenticating as a Service Principal not all required information is available
As such instead we support authenticating as a Service Principal using the separate auth methods
for Client Certificate/Secret authentication.

```
$ go test -v ./authentication/
=== RUN   TestAzureCLITokenAuth_isApplicable
--- PASS: TestAzureCLITokenAuth_isApplicable (0.00s)
=== RUN   TestAzureCLITokenAuth_populateConfig
--- PASS: TestAzureCLITokenAuth_populateConfig (0.00s)
=== RUN   TestAzureCLITokenAuth_validate
--- PASS: TestAzureCLITokenAuth_validate (0.00s)
=== RUN   TestServicePrincipalClientCertAuth_builder
--- PASS: TestServicePrincipalClientCertAuth_builder (0.00s)
=== RUN   TestServicePrincipalClientCertAuth_isApplicable
--- PASS: TestServicePrincipalClientCertAuth_isApplicable (0.00s)
=== RUN   TestServicePrincipalClientCertAuth_populateConfig
--- PASS: TestServicePrincipalClientCertAuth_populateConfig (0.00s)
=== RUN   TestServicePrincipalClientCertAuth_validate
--- PASS: TestServicePrincipalClientCertAuth_validate (0.00s)
=== RUN   TestServicePrincipalClientSecretAuth_builder
--- PASS: TestServicePrincipalClientSecretAuth_builder (0.00s)
=== RUN   TestServicePrincipalClientSecretAuth_isApplicable
--- PASS: TestServicePrincipalClientSecretAuth_isApplicable (0.00s)
=== RUN   TestServicePrincipalClientSecretAuth_populateConfig
--- PASS: TestServicePrincipalClientSecretAuth_populateConfig (0.00s)
=== RUN   TestServicePrincipalClientSecretAuth_validate
--- PASS: TestServicePrincipalClientSecretAuth_validate (0.00s)
=== RUN   TestManagedServiceIdentity_builder
2019/05/23 12:49:55 [DEBUG] Using MSI endpoint "https://hello-world"
--- PASS: TestManagedServiceIdentity_builder (0.00s)
=== RUN   TestManagedServiceIdentity_isApplicable
--- PASS: TestManagedServiceIdentity_isApplicable (0.00s)
=== RUN   TestManagedServiceIdentity_populateConfig
--- PASS: TestManagedServiceIdentity_populateConfig (0.00s)
=== RUN   TestManagedServiceIdentity_validate
--- PASS: TestManagedServiceIdentity_validate (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_Expired
--- PASS: TestAzureFindValidAccessTokenForTenant_Expired (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_ExpiringIn
--- PASS: TestAzureFindValidAccessTokenForTenant_ExpiringIn (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_InvalidManagementDomain
2019/05/23 12:49:55 [DEBUG] Resource "https://portal.azure.com/" isn't a management domain
--- PASS: TestAzureFindValidAccessTokenForTenant_InvalidManagementDomain (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_DifferentTenant
2019/05/23 12:49:55 [DEBUG] Resource "https://management.core.windows.net/" isn't for the correct Tenant
--- PASS: TestAzureFindValidAccessTokenForTenant_DifferentTenant (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_Valid
--- PASS: TestAzureFindValidAccessTokenForTenant_Valid (0.00s)
=== RUN   TestAzureFindValidAccessTokenForTenant_NoTokens
--- PASS: TestAzureFindValidAccessTokenForTenant_NoTokens (0.00s)
=== RUN   TestAzureCliProfile_populateSubscriptionIdMissing
--- PASS: TestAzureCliProfile_populateSubscriptionIdMissing (0.00s)
=== RUN   TestAzureCliProfile_populateSubscriptionIdNoDefault
--- PASS: TestAzureCliProfile_populateSubscriptionIdNoDefault (0.00s)
=== RUN   TestAzureCliProfile_populateSubscriptionIdValid
--- PASS: TestAzureCliProfile_populateSubscriptionIdValid (0.00s)
=== RUN   TestAzureCliProfile_populateTenantIdEmpty
--- PASS: TestAzureCliProfile_populateTenantIdEmpty (0.00s)
=== RUN   TestAzureCliProfile_populateTenantIdMissingSubscription
--- PASS: TestAzureCliProfile_populateTenantIdMissingSubscription (0.00s)
=== RUN   TestAzureCliProfile_populateTenantIdValid
--- PASS: TestAzureCliProfile_populateTenantIdValid (0.00s)
=== RUN   TestAzureCLIProfileFindDefaultSubscription
--- PASS: TestAzureCLIProfileFindDefaultSubscription (0.00s)
=== RUN   TestAzureCLIProfileFindSubscription
--- PASS: TestAzureCLIProfileFindSubscription (0.00s)
=== RUN   TestAzureEnvironmentNames
--- PASS: TestAzureEnvironmentNames (0.00s)
PASS
ok  	github.com/hashicorp/go-azure-helpers/authentication	1.319s
```

authentication/auth_method_azure_cli_token.go

@@ -38,6 +38,12 @@ func (a azureCliTokenAuth) build(b Builder) (authMethod, error) {
 
 	auth.profile.profile = profile
 
+	// Authenticating as a Service Principal doesn't return all of the information we need for authentication purposes
+	// as such Service Principal authentication is supported using the specific auth method
+	if authenticatedAsAUser := auth.profile.verifyAuthenticatedAsAUser(); !authenticatedAsAUser {
+		return nil, fmt.Errorf("Authenticating using the Azure CLI is only supported as a User (not a Service Principal)")
+	}
+
 	err = auth.profile.populateFields()
 	if err != nil {
 		return nil, fmt.Errorf("Error retrieving the Profile from the Azure CLI: %s Please re-authenticate using `az login`.", err)

authentication/azure_cli_profile.go

@@ -1,6 +1,8 @@
 package authentication
 
 import (
+	"strings"
+
 	"github.com/Azure/go-autorest/autorest/azure/cli"
 )
 
@@ -33,3 +35,18 @@ func (a *azureCLIProfile) populateFields() error {
 	// always pull the environment from the Azure CLI, since the Access Token's associated with it
 	return a.populateEnvironment()
 }
+
+func (a *azureCLIProfile) verifyAuthenticatedAsAUser() bool {
+	for _, subscription := range a.profile.Subscriptions {
+		if subscription.User == nil {
+			continue
+		}
+
+		authenticatedAsAUser := strings.EqualFold(subscription.User.Type, "user")
+		if authenticatedAsAUser {
+			return true
+		}
+	}
+
+	return false
+}