ceph-disk: enable directory backed OSD at boot time

ceph-disk: enable directory backed OSD at boot time

https://github.com/ceph/ceph/commit/539385b143feee3905dceaf7a8faaced42f2d3c6
introduced a regression preventing directory backed OSD from starting at
boot time.

For device backed OSD the boot sequence starts with ceph-disk@.service
and proceeds to

    systemctl enable --runtime ceph-osd@.service

where the --runtime ensure ceph-osd@12 is removed when the machine
reboots so that it does not compete with the ceph-disk@/dev/sdb1 unit at
boot time.

However directory backed OSD solely rely on the ceph-osd@.service unit
to start at boot time and will therefore fail to boot.

The --runtime flag is selectively set for device backed OSD only.

Fixes: http://tracker.ceph.com/issues/19628
Signed-off-by: Loic Dachary <loic@dachary.org>