Fix user with missing current group migration

cc @brandondunne @Fryguy @lfu @dajohnso 

Thanks @Fryguy for helping pairing/reviewing on this one.

To address https://bugzilla.redhat.com/show_bug.cgi?id=1145794
We added a test case for a user in a current group that is not in the the miq_groups list and further clarified the orphaned current_group_id test case.