improve lxc, use cirros images rather than ttylinux

See commit messages for more verbosity.  These 4 commits do:
 * use cgroup-lite package for cgroup mounts
 * fix bug in LIBVIRT_TYPE=lxc (it would actually just fail)
 * make IMAGE_URLS able to contain .img or .img.gz files
 * use cirros 0.3.0 images for "tiny image"

The cirros is probably the most intrusive as I'm sure people are accostomed to the 'ubuntu@' and password for ttylinux-uec.  In cirros, you need 'cirros@' and default password is 'cubswin:)'