When deploying the ceph-fs-mirror daemon we don't need to name the
deployment like "rook-ceph-fs-mirror-a", we can simply go with
"rook-ceph-fs-mirror". Today, in Pacific, the daemon only supports a
single replica. In the future, the Ceph Quincy release should support
mulitiple concurrent daemons, at this point Rook will introduce a
"count" field in the CephFilesystemMirror CRD specification. Internally,
this field will map with the "replica" value of the Deployment.
Signed-off-by: Sébastien Han <seb@redhat.com>