MiqWidget.available_for_user should check by both role and group.

MiqWidget visibility can be set either by role or by group. 
Currently MiqWidget.available_for_user checks only by role. So widget visibility does not work for group.

https://bugzilla.redhat.com/show_bug.cgi?id=1158636
@lfu Let's correct the comment for this commit and create a second commit on this PR to correct the condition statement.