Changes to show tag category only once for each base class.

- Changed TAG_CLASSES to be a hash and then use invert on it in order to use each model only once to add tag categories for them, having base class more than once in that array was causing duplicate entries in the pull down.
- Added spec test for model_details method to make sure values in the pull down are uniq.

https://bugzilla.redhat.com/show_bug.cgi?id=1160209