enable ceph-mon.target service after monitor start

enable ceph-mon.target service after monitor start

ceph-create-keys unit file was removed here:

* https://github.com/ceph/ceph/commit/8bcb4646b6b9846bb965cdec3ca2a21eb3b26bab
* https://github.com/ceph/ceph/commit/dc5fe8d415858358bd0baf5d8dce0a753f5e0cea

As a consequence the systemctl preset command now fails to run since the
unit does not exist anymore. Due to the redirection in /dev/null we
don't know what's happening.

Ultimately the mon unit doesn't get enabled and the mon service won't
start after reboot.
Removing the old/non-existent unit makes the command succeed now.

ceph fix: https://github.com/ceph/ceph/pull/14226

Signed-off-by: WingkaiHo <sanguosfiang@163.com>
Co-Authored-By: Sébastien Han <seb@redhat.com>