Prevent error message when creating a user

keystone changed behaviour and didn't add the user
to the specified role. Now it does; this patch
prevents an exception popping up without breaking
with previous versions.

Change-Id: I115a5126b70ae3c7733aa065bf55104f88cc42e5
Closes-Bug: #1260439
(cherry picked from commit f5ded29d9a73db4c225cbfb664e7d85e5aa0996b)
Merge "Prevent error message when creating a user" into stable/icehouse