core: set active image after Live Merge of active layer

Live Merging the active layer should mark the new active image.

Change-Id: Ifc2777257b85c52011aa3bd7c9ad56c2ea680b63
Bug-Url: https://bugzilla.redhat.com/1122691
Related-To: https://bugzilla.redhat.com/1083310
Signed-off-by: Greg Padgett <gpadgett@redhat.com>