Fix UI vm summary performance (part 2)

Move RBAC descendant check to TreeBuilder to consolidate duplicate code.

Follow up to https://github.com/ManageIQ/manageiq/pull/81
https://bugzilla.redhat.com/show_bug.cgi?id=1086015

It turns out the code I fixed in #81 had a straight copy-paste duplicate in TreeBuilder.  So, I consolidated the two, and made the explorer.rb one call the TreeBuilder one.

@dclarizio Please review.