rbd-mirror: default <-> non-default namespace remapping

rbd-mirror: default <-> non-default namespace remapping

These changes allow the pool to be configured to disable mirroring on
the default namespace if required (init-only mode). It also allows the
default namespace to be mirrored to a non-default namespace on the
remote pool.

Co-authored-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: N Balachandran <nibalach@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>