Add support for ubuntu

Not sure if this is the best way to integrate ubuntu support into the cookbook, but it works as it is.
Willing to take advice on integrations.
Shouldn't this be downloaded from `node[:stackdriver][:repo_url]` instead? If not then the change you made to support Amazon Linux (which I want badly!) won't do anything as the stackdriver.repo file in this cookbook is the one for the non-Amazon RHEL variants.