Fix US GovCloud AD endpoint for AZURE_ARM provider

Use active directory endpoint from `az cloud list` output.

Author: CrCliff

libcloud/common/azure_arm.py

@@ -101,7 +101,7 @@ def parse_error(self):
         "sqlManagementEndpointUrl": "https://management.core.usgovcloudapi.net:8443/",
         "sqlServerHostnameSuffix": ".database.usgovcloudapi.net",
         "galleryEndpointUrl": "https://gallery.usgovcloudapi.net/",
-        "activeDirectoryEndpointUrl": "https://login-us.microsoftonline.com",
+        "activeDirectoryEndpointUrl": "https://login.microsoftonline.us",
         "activeDirectoryResourceId": "https://management.core.usgovcloudapi.net/",
         "activeDirectoryGraphResourceId": "https://graph.windows.net/",
         "activeDirectoryGraphApiVersion": "2013-04-05",