cluster-merge: implement params delta mercifulness

Sometimes it's good to tell the user about parameter differences but
then proceed anyway. Strictness is still enforced for those parameters
that would break the cluster (volume group name, storage dir if file
storage is enabled).

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>