Fix invalid credential error handling in authentication process

[Didainius]

This PR builds on top of vmware/go-vcloud-director#536 and only adds additional tests as the real fix is in go-vcloud-director SDK.

We had an unhandled HTTP 401 (Unauthorized) error in the password authentication flow and as a result it would not return an error when user supplies invalid credentials. The errors then become very confusing and the user is left helpless to do his guesswork.

Before, this would cause very confusing errors similar to:

Error: could not find any extended Provider VDC with name ext-net-name: error retrieving query: API Error: 406: The request has invalid accept header: Invalid API version requested. Supported API versions are: [37.0.0-alpha, 36.3, 36.2, 36.1, 36.0, 35.2, 35.0, 34.0, 33.0 [D], 32.0 [D], 31.0 [D]] ([D] indicates deprecated versions)
│
│   with data.vcd_provider_vdc.nsxt-pvdc,
│   on Org-creation-Main.tf line 47, in data "vcd_provider_vdc" "nsxt-pvdc":
│   47: data "vcd_provider_vdc" "nsxt-pvdc" {
│
╵
╷
│ Error: could not find external network V2 by name 'ext-net-name': could not find external network by name: error getting all pages for endpoint https://HOSTNAME/cloudapi/1.0.0/externalNetworks/: error in HTTP GET request: NOT_ACCEPTABLE - The request has invalid accept header: Invalid API version requested. Supported API versions are: [37.0.0-alpha, 36.3, 36.2, 36.1, 36.0, 35.2, 35.0, 34.0, 33.0 [D], 32.0 [D], 31.0 [D]] ([D] indicates deprecated versions)
│
│   with data.vcd_external_network_v2.nsxt-ext-net,
│   on Org-creation-Main.tf line 98, in data "vcd_external_network_v2" "nsxt-ext-net":
│   98: data "vcd_external_network_v2" "nsxt-ext-net" {

After the fix, the user would get similar error which directly hints that one should check his credentials:

   auth_test.go:310: Step 1/1 error: Error running pre-apply refresh: exit status 1

        Error: something went wrong during authentication: error authorizing: received response HTTP 401 (Unauthorized). Please check if your credentials are valid

          with provider["registry.terraform.io/hashicorp/vcd"],
          on terraform_plugin_test.tf line 2, in provider "vcd":
           2:                   provider "vcd" {

Three additional subtests are added to TestAccAuth that expect an error:

• TestAccAuth/InvalidSystemUserAndPasswordWithDefaultOrgAndVdc
• TestAccAuth/InvalidPassword,AuthType=integrated
• TestAccAuth/InvalidSystemUserAndPassword,AuthType=integrated

[adambarreiro]

LGTM!

[lvirbalas]

Just one suggestion for changelog.