Restore the ability to load external templates
Restore the ability to load external templates

Change the Theme Template loader to not absolutely require that
templates reside under the Django directory (openstack_dashboard), by
deferring the usage of safe_join to the point where that check is
made and handled correctly.

Also addressed on Pythonic nit noted in the review of
https://review.openstack.org/317051

Closes-Bug: #1680265
Change-Id: Icf86df5e213d47b207e99b721e9e0fcdc898882e

noironetworks/horizon,BiznetGIO/horizon,openstack/horizon,noironetworks/horizon,yeming233/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,NeCTAR-RC/horizon,ChameleonCloud/horizon,ChameleonCloud/horizon,openstack/horizon,openstack/horizon,noironetworks/horizon,yeming233/horizon,ChameleonCloud/horizon,NeCTAR-RC/horizon,BiznetGIO/horizon,BiznetGIO/horizon,BiznetGIO/horizon,noironetworks/horizon,yeming233/horizon,openstack/horizon,yeming233/horizon,NeCTAR-RC/horizon