postgresql: Update setup-disks to use `service postgresql`.

Using `service postgresql` makes it no longer linked to the specific
version/cluster that is on the host.

puppet/zulip_ops/files/postgresql/setup_disks.sh

@@ -7,10 +7,8 @@ mkfs.xfs $LOCALDISK
 echo "$LOCALDISK   /srv  xfs    nofail,noatime 1 1" >> /etc/fstab
 mount /srv
 
-# TODO use systemctl instead of pg_ctlcluster on CentOS
-pg_ctlcluster 9.5 main stop
+service postgresql stop
 mv /var/lib/postgresql /srv
 ln -s /srv/postgresql/ /var/lib
 
-# TODO use systemctl instead of pg_ctlcluster on CentOS
-pg_ctlcluster 9.5 main start
+service postgresql start