google compute engine integration

Initial google compute engine integration.

- includes
  * ability to provision google compute instances from google compute images
  * since this uses google compute pre-built images only supports debian + centos
  * fixes to existing spec tests
  * update to README to cover new hypervisor

- missing new spec tests to cover new google hypervisor
- if we start to create our own google compute images some of the set up steps will need to be updated (right now images are pulled from the centos-cloud and debian-cloud projects)