This patch adds several options to allow using OpenVZ virt layer.
- modifies stack.sh and stackrc to recognize a new VIRT_TYPE option
- set IMAGE_URLS to an openvz image, if VIRT_TYPE == openvz
It also makes a few changes to some tests so that some implicit
defaults (such as the guest user account) can be overridden.
Change-Id: I0dde2dffbf3848fac1dd27eb37af84c0ac73d9aa