Set image location explicitly

It's ideal for the image location in mediawiki to be outside of
its path. This allows you to have multiple copies of the software
while having the uploads in a shared spot.

Change-Id: Iae0ac3e13213353d6f101c62f5e150cf844b5694
Reviewed-on: https://review.openstack.org/17575
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins