cephadm: create /var/run/ceph dir via unit.run, not unit file

cephadm: create /var/run/ceph dir via unit.run, not unit file

The systemd unit file is shared with non-ceph daemons, which (1) don't
need the /var/run directory, and (2) are based on a uid/gid from a
different container image, which means we can't figure out the right
ceph uid/gid from them to set the ownership properly.

Instead, put it in the unit.run file... and only for ceph daemons when
we have the uid/gid we need.

Fixes: https://tracker.ceph.com/issues/44894
Signed-off-by: Sage Weil <sage@redhat.com>