keystone_user_role fix for when user not granted roles yet

Fix situation where keystone_user_role resource fails if the
user is not already granted at least one role on the tenant.

Change-Id: I95e3e0aac784d0bc70f0c8a8db2cc13d74a174ea
Closes-Bug: 1381725
Merge "keystone_user_role fix for when user not granted roles yet"