detect latest version of coreos

The latest version of coreos doesn't include the `/etc/gentoo-release` file which the current coreos guest is using to detect that the OS is coreos.

It's been renamed to `/etc/os-release`.

This PR adds support for the latest coreos, as well as providing backwards compatibility for previous releases.

A virtualbox image with the new file change can be downloaded at: http://storage.core-os.net/coreos/amd64-generic/228.0.0/coreos_production_vagrant.box