Support for Google-Containers

Recently Google have released a google VM Image that comes with docker preinstalled and supports docker provisioning via metadata.

Toady in libcloud both debian and centos images are supported by matching against the VM name and then injecting the appropiate project name (`debian-cloud` and `centos-cloud`).

This adds support for `google-containers`.

Closes #299

Signed-off-by: Tomaz Muraus <tomaz@apache.org>