core: Allow decreasing a cluster compatability version

Under some circumstences a cluster can decrease a compatability version.
This patch supports this.

Change-Id: I2a8f52c63a82871e4ac1847f0414c82095f43075
Bug-Url: https://bugzilla.redhat.com/1076555
Signed-off-by: lzelkha@redhat.com <lzelkha@redhat.com>