Allowing default KUBECONFIG ordering

This commit aims to make aware `eksctl` of the `KUBECONFIG` environment
variable, especially during cluster creation and kubeconfig retrieval
via the `eksctl utils write-kubeconfig` command.
To avoid conflicts and keep the actual convention, the variable
`DefaultPath` has been declared using a simple _func_ in order to don't
perform an invasive refactoring requested by a switch to function
declaration.
Actually, using the standard library to retrieve the environment
variable and not using the `viper` helper (although already a dependency)
since the `KUBECONFIG` one is considered as a global one, so totally
unbounded from `eksctl`.
Code coverage is going to decrease since during the test suite setup,
the environment variable is evaluated before the execution of tests: to
provide further coverage, the conversion from variable to function has
to be performed, although this requires some changes in the following
files and should be considered and approved by maintainers:
- pkg/ctl/cmdutils/cmdutils.go
- pkg/ctl/create/cluster.go
- pkg/ctl/utils/write_kubeconfig.go
- pkg/utils/kubeconfig/kubeconfig.go

Closes eksctl-io#228

pkg/utils/kubeconfig/kubeconfig.go

@@ -18,15 +18,22 @@ import (
 )
 
 // DefaultPath defines the default path
-var DefaultPath = clientcmd.RecommendedHomeFile
+var DefaultPath = func(env string) string {
+	if len(env) > 0 {
+		return env
+	}
+	return clientcmd.RecommendedHomeFile
+}(os.Getenv(RecommendedConfigPathEnvVar))
 
 const (
 	// AWSIAMAuthenticator defines the name of the AWS IAM authenticator
 	AWSIAMAuthenticator = "aws-iam-authenticator"
 	// HeptioAuthenticatorAWS defines the old name of AWS IAM authenticator
 	HeptioAuthenticatorAWS = "heptio-authenticator-aws"
 	// AWSEKSAuthenticator defines the recently added `aws eks get-token` command
-	AWSEKSAuthenticator = "aws"
+	AWSEKSAuthenticator         = "aws"
+	// Shadowing the default kubeconfig path environment variable
+	RecommendedConfigPathEnvVar = clientcmd.RecommendedConfigPathEnvVar
 )
 
 // AuthenticatorCommands returns all of authenticator commands