Allow one to specify horizon ALLOWED_HOSTS

Allow one to specify horizon ALLOWED_HOSTS

If horizon is running in production (DEBUG is False), it will answer
only to the IPs/hostnames specified in the ALLOWED_HOSTS variable in the
local_settings.py configuration file.

The puppet-horizon module offer the feature to customize that,
tripleo-heat-teamplates was missing the link between the top-level
parameter and the puppet parameter, hence this commit.

More info :

 * https://docs.djangoproject.com/en/dev/ref/settings/#allowed-hosts
 * https://github.com/openstack/puppet-horizon/blob/master/templates/local_settings.py.erb#L14-L24

Change-Id: I5faede8b74a0318e15baa761dc502b95b051ae0d