Use a cirros image as the end user image

For testing, there is no need to have a full $NODE_DIST image.
Using a cirros image, the memory footprint (both memory and disk) is
smaller and there is no need to create the image using dib.

This can be activated with the env. variable USE_CIRROS=1

Change-Id: Id0e6177068644ca2da3ccc30ca33e3c9424687eb
Co-Authored-By: Robert Collins <rbtcollins@hp.com>
Merge "Use a cirros image as the end user image"