Allow elements to include skeleton config:

This code provides 'use-config-templates' for elements
to use to install os-config-applier templates to
/opt/stack/openstack-config-templates.

These templates will be filled in from heat
metadata by os-config-applier upon first boot.

This change updates keystone to install templates
in this way as an example.

Change-Id: I0be0a79a431e9ba5b80e84f130c48d5ce8b100ae