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
Or maybe just change this spec to say:

```ruby
user   = user_stub.create!(:current_group => group2, :miq_groups => [group1])
```