docs(gce): Instructions for running Deis in Google Compute Engine

This is some preliminary instructions for running Deis on Google Compute Engine. I adapted it slightly from https://gist.github.com/andyshinn/a78b617b2b16a2782655 to include a `create-gce-user-data` script which re-uses the existing `contrib/coreos/user-data` as a base and adds ephemeral storage formatting to store Docker data on a volume separate from root.

There is one change to existing `contrib/coreos/user-data` which may want a second pair of eyes. The permissions for the `write_files` section were being converted by PyYAML as octal (0755 would become 493). I changed this to be a string which seems to work just fine in CoreOS cloud-init.

I'm not a programmer and don't pretend to be one. Feel free to comment on ways I might improve (or reuse existing Deis Python libs) the Python script!
I know that we'd like to use it, but maybe we should discuss this. I know that we have mainline support for EC2, Rackspace (when they actually release a newer version of CoreOS) and DigitalOcean, but do we want to say that OpDemand officially supports GCE? Maybe we can say there is third party support for it? ping @carmstrong @mboersma