adopt: fix rbd mirror adoption

adopt: fix rbd mirror adoption

The rbd mirroring is broken because cephadm doesn't bindmount /etc/ceph anymore.
It means the keyrings and ceph config file aren't available after the
migration.
The idea here is to remove the current rbd mirror peer and add it back
to the mon config store so we aren't bound to the /etc/ceph directory.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1967440
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 9c794aa9bcfab177f437f503bda0ff37dceca319)