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
What happens to user.miq_groups when user's current_group is deleted and nullified? Is the deleted group still left behind in user.miq_groups?