better systemd support

Added ceph.target
Made ceph-mds, ceph-mon, ceph-osd services part of ceph.target
Made ceph-mds, ceph-mon, ceph-osd services require partitions to be available.
Added ceph init script with sysV like behaviour.

Provided by Tim Serong <tserong@suse.com> and Owen Synge <osynge@suse.com>

Signed-off-by: Owen Synge <osynge@suse.com>