toaster: Update oe-core fixture for Morty release

toaster: Update oe-core fixture for Morty release

Update the oe-core fixture to the Morty release.  This removes the
master branch from the release and limits it to the morty branch.
Normally, we would also support at least one past branch but the change
from Python 2.7 -> Python 3 makes that infeasible.

Signed-off-by: brian avery <brian.avery@intel.com>
Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>