Added ability to set custom local_setttings.py.erb template path for hor...

The default template in the horizon module only allows for connections from the local host via ALLOWED_HOSTS. With no way of using a custom path, any local changes get overwritten every time the manifest is applied. 

Simply exposed in openstack/horizon and openstack/controller what is available in horizon/init. By default, everything will continue to work as it did before. However now if desired, a manifest implementing openstack/controller can pass in a path to a custom local_settings file template.