Allow selection of different kickstart variants and/or repos
As part of https://github.com/Katello/katello/pull/4282 we added the restriction of a single release/arch for kickstart trees in a content view.
We need to decide if:
a) we should allow only different variants of release & arch (i.e. Desktop & Server 6.4 x86_64)
b) if we should allow any arbitrary kickstart trees and let the user select which one they want based on the host's release and arch