[Fedora-spins] Spins and branching

Now that we have branched for F15 I changed the default repos use in the ks files (livebase, mini and aos) to use F15 (via $releasever). Neither updates (since there isn't an updates repo until close to release) nor updates-testing have been enabled. Note the spin-kickstarts repo hasn't been branched. We don't typically do that until very close to release, since almost all development now is targetting the F15 release. I expect the nightly composes to switch over to building against the F15 repo repo shortly. I'll be doing another spin-kickstarts build before alpha so that the F15 repos are referred to by default on the alpha.