[Fedora-spins] spin-kickstarts branched for F15

Any changes destined for the F15 release should now be committed in the F-15 branch of spin-kickstarts. Changes destined for F16 should be commited to the master branch of spin-kickstarts. Once final freeze starts (May 9th), changes to the F-15 branch should only be made to fix NTH or blocker bugs. The nightly composes should be switched over to using the the F-15 branch until a bit after the F15 release.