Prevent stale data in LDAP group sync last owner check
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17764
Customers were reporting that LDAP group sync would occasionally remove the last owner of a group. This didn't seem right (we have tests for this) so I dug in to see what was going on. Previously owners
was a memoized method which was the reason for stale data. Now, it's an AR association and the same problem applies. The easiest solution is to reload the model before checking for last owner.