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

Added changes to store value of TAG_CLASSES array into 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.

https://bugzilla.redhat.com/show_bug.cgi?id=1160209

@dclarizio please review & test.
list of tags is being built using constantize.base_class on for example "VmInfra" which will be vm, and for any the other ones on that list such as Vm, VmOrTemplate, VmCloud. I think it shouldn't matter if invert gives me the last one here.