Set nullglob in tools/partition-disks

So that this script can be run on LXC machines that don't have any "SCSI
disk" devices. Otherwise the glob doesn't get expanded and we try to run
`parted … /dev/sd?`. Example:

    vagrant@postgresql-slave-1:~$ for disk in /dev/sd?; do echo $disk; done
    /dev/sd?
    vagrant@postgresql-slave-1:~$ shopt -s nullglob
    vagrant@postgresql-slave-1:~$ for disk in /dev/sd?; do echo $disk; done

tools/partition-disks

@@ -1,5 +1,7 @@
 #!/bin/bash
 set -e
+shopt -s nullglob
+
 for disk in /dev/sd?; do
   echo "checking ${disk}"
   if ! sudo parted -a optimal --script  ${disk} -- print >/dev/null