microsoft: improve azure active directory endpoint

Rename AzureActiveDirectoryEndpoint to AzureADEndpoint. Add default tenant ("common").
Delete azure package (azure.Endpoint duplicates microsoft.AzureADEndpoint).

Change-Id: I48a7679ffddd984f5744a64edfa4df958c18ed66
Reviewed-on: https://go-review.googlesource.com/83696
Reviewed-by: JBD <[email protected]>

Author: rumyantseva

azure/azure.go

@@ -15,11 +15,15 @@ var LiveConnectEndpoint = oauth2.Endpoint{
 	TokenURL: "https://login.live.com/oauth20_token.srf",
 }
 
-// AzureActiveDirectoryEndpoint returns a new oauth2.Endpoint for the given tenant at Azure Active Directory.
+// AzureADEndpoint returns a new oauth2.Endpoint for the given tenant at Azure Active Directory.
+// If tenant is empty, it uses the tenant called `common`.
 //
 // For more information see:
 // https://docs.microsoft.com/en-us/azure/active-directory/develop/active-directory-v2-protocols#endpoints
-func AzureActiveDirectoryEndpoint(tenant string) oauth2.Endpoint {
+func AzureADEndpoint(tenant string) oauth2.Endpoint {
+	if tenant == "" {
+		tenant = "common"
+	}
 	return oauth2.Endpoint{
 		AuthURL:  "https://login.microsoftonline.com/" + tenant + "/oauth2/v2.0/authorize",
 		TokenURL: "https://login.microsoftonline.com/" + tenant + "/oauth2/v2.0/token",