Add explicit mmap addrs for varnish persistent storage

This should eliminate failed varnish restarts due to
mmap failures in the long run.

Note the addresses only take effect on newly-allocated storage,
so we won't see the benefits on a given machine until we wipe its
persistent cache for one reason or another and let varnish
recreate it.

Requires a varnish package with the patch for the new arg:
 * 3.0.6-plus~x-wm2 or higher
 * 3.0.5-plus~x-wm7 or higher

Change-Id: Ie55f50f4a60097f9eba1b9dc3d880ba848333cb2