Fix gov us cloud definition (#105)

Author: lmazuel

msrestazure/azure_cloud.py

@@ -170,7 +170,7 @@ def __str__(self):
         sql_management='https://management.core.usgovcloudapi.net:8443/',
         batch_resource_id='https://batch.core.usgovcloudapi.net/',
         gallery='https://gallery.usgovcloudapi.net/',
-        active_directory='https://login-us.microsoftonline.com',
+        active_directory='https://login.microsoftonline.us',
         active_directory_resource_id='https://management.core.usgovcloudapi.net/',
         active_directory_graph_resource_id='https://graph.windows.net/'),
     suffixes=CloudSuffixes(