Feature #7652 - CoreOS support

Implements foreman CoreOS OperatingSystem Support (http://projects.theforeman.org/issues/7652). PXE and cloud-config Templates will be send in a different PR to the community-template repository.

since this method already exists in the parent class, cant you do something like
```ruby
def boot_files_uri(..)
 super(..).gsub.. ?
end
```