init-ceph: wrap daemon startup with systemd-run when running under systemd

We want to make sure the daemon runs in its own systemd environment.  Check
for systemd as pid 1 and, when present, use systemd-run -r <cmd> to do
this.

Probably fixes #7627

Signed-off-by: Sage Weil <sage@redhat.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>
Tested-by: Dan Mick <dan.mick@inktank.com>