Allow to disable MDS

Even if MDS are not configured in site.yml the playbook has a
dependancy on the ceph.conf template.
This disables the mds section from the ceph.conf file.

Closes: #21

Signed-off-by: Sébastien Han <sebastien.han@enovance.com>
Merge pull request #22 from leseb/fix-allow-disable-mds

Allow to disable MDS