Set a more generous default image cache size

Master images of non-trivial filesystems will run 3+G (even though
the qcow2 might be < 1G) so we need to make it big enough to be useful
:). Ideally we'd autotune this based on free space or something, but
for now, this is large enough to cache e.g. TripleO cluster workloads
but small enough to fit in test VMs. This patch sets the default cache
to 20G.

Change-Id: I11ca629589e8a90f727abcc5b1c7e72dbd9e4b0f
Merge "Set a more generous default image cache size"