rgw: run radosgw as apache with systemd

Same as sysv.

http://tracker.ceph.com/issues/10125 Fixes: #10125

Signed-off-by: Loic Dachary <loic@dachary.org>
Merge pull request #3059 from dachary/wip-10125-radosgw-init

rgw: run radosgw as apache with systemd

Reviewed-by: Sage Weil <sage@redhat.com>