[Fedora-spins] Spins and branching

[Bruno Wolff III]

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.