ceph-config: do not log cluster log on container

ceph-config: do not log cluster log on container

The container image recently merged both cluster and mon log into a
single stream. Following this, we now see this warning coming from the
container image:

2018-06-19 13:44:01.542990 7ff75b024700  1 mon.vm02@1(peon).log
v57928205 unable to write to '/var/log/ceph/ceph.log' for channel
'cluster': (2) No such file or directory

So we now tell the mon to not log cluster log on the filesystem.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1591771
Signed-off-by: Sébastien Han <seb@redhat.com>