Fixes #2189: environment defaults to Azure Public Cloud

Ideally this would be drawn from Vault Server, but the documented
default there is to point at Azure Public Cloud. This seems like a
decent compromise.

Signed-off-by: David Alexander <opensource@thelonelyghost.com>

vault/data_source_azure_access_credentials.go

@@ -313,6 +313,9 @@ func azureAccessCredentialsDataSourceRead(ctx context.Context, d *schema.Resourc
 }
 
 func getAzureCloudConfigFromName(name string) (cloud.Configuration, error) {
+	if c == "" {
+		return cloud.AzurePublic, nil
+	}
 	if c, ok := azureCloudConfigMap[strings.ToUpper(name)]; !ok {
 		return c, fmt.Errorf("unsupported Azure cloud name %q", name)
 	} else {