Update devtestrc variable values

Some variables can and should be kept at their defaults now, so they
are not being assigned in devtestrc anymore.

Increasing VM disk size to increase /mnt/state size. The VM images are
qcow2 so they won't take up too much space anyway. With 20 GB (as set
previously) deployment can fail because undercloud ironic doesn't have
enough space for images.

Change-Id: Ib275878c494f01f08b1ad04501d1c4cac3bc17ba