Running instructions do not contain non-commercial Azure cloud support

[agray0232]

Describe the bug
The azure examples do not contain instructions sufficient for running the tests in non-commercial (ie USGov) cloud environments. "AZURE_ENVIRONMENT" needs to be set as an environment variable for any non-commercial cloud test to work.

Running a simple test case for a USGov cloud deployments results in a "Subscription not found" 404 error thrown by the resource_client attempting to retrieve the test resources for evaluation.

The client_factory has documented the environment variable it uses to know what Azure cloud to use when generating resource clients.

To successfully test the deployment of resources in a non-commercial cloud environment, the "AZURE_ENVIRONMENT" environment variable needs to be set by the user.

To Reproduce

• Set your active cloud with the Azure CLI to non-commercial: az cloud set --name AzureUSGovernment
• Login to your azure account: az login
• Run TestTerraformAzureExample from test/azure/terraform_azure_example_test.go
• The test will fail with a 404 error saying your subscription was not found
• Set the AZURE_ENVIRONMENT variable: export AZURE_ENVIRONMENT=AzureUSGovernmentCloud
• Run TestTerraformAzureExample from test/azure/terraform_azure_example_test.go
• The test should pass

Error Trace

    resourcegroup.go:16: 
        	Error Trace:	/Users/<user>/go/pkg/mod/github.com/gruntwork-io/terratest@v0.44.0/modules/azure/resourcegroup.go:16

        	Error:      	Received unexpected error:
        	            	resources.GroupsClient#Get: Failure responding to request: StatusCode=404 -- Original Error: autorest/azure: Service returned an error. Status=404 Code="SubscriptionNotFound" Message="The subscription '<subscription_id>' could not be found."

Expected behavior
Tests should pass for non-commercial Azure clouds

Versions

• Terratest version: v0.44.0