Update docker driver to use a CirrOS image

For purposes of matching the VM image used
in devstack across hypervisors, set the
default container image for Docker to cirros.

This uses the CirrOS image from stackbrew,
the "standard library" for Docker.

Change-Id: I9d767a4e06c5caa7b92ffea25e6a9aeda9bf282a
Merge "Update docker driver to use a CirrOS image"