Add support for setting OPENSTACK_ENDPOINT_TYPE

Horizon's OPENSTACK_ENDPOINT_TYPE [defaults to "publicURL"](https://github.com/openstack/horizon/blob/stable/grizzly/openstack_dashboard/api/base.py#L152-154), despite [the documentation](https://github.com/openstack/horizon/blob/stable/grizzly/openstack_dashboard/local/local_settings.py.example#L133-136). These commits update the documentation in our local_settings.py as well as create a node attribute to override it as needed.