mds: allow to disable [mds] section in ceph.conf if not needed

Added new parameter to ceph::conf (mds_activate) to disable
the [mds] section of ceph.conf if not needed.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
ceph_conf_spec.rb: adapt to new param mds_activate

Added test for default param+mds_activate=false.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>