rgw/rbdmirror: use service dump instead of ceph -s

rgw/rbdmirror: use service dump instead of ceph -s

The ceph status command returns a lot of information stored in variables
and/or facts which could consume resources for nothing.
When checking the rgw/rbdmirror services status, we're only using the
servicmap structure in the ceph status output.
To optimize this, we could use the ceph service dump command which contains
the same needed information.
This command returns less information and is slightly faster than the ceph
status command.

$ ceph status -f json | wc -c
2001
$ ceph service dump -f json | wc -c
1105
$ time ceph status -f json > /dev/null

real	0m0.557s
user	0m0.516s
sys	0m0.040s
$ time ceph service dump -f json > /dev/null

real	0m0.454s
user	0m0.434s
sys	0m0.020s

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>