Custom themes support for horizon deployment.
 - horizon role task added for copying theme files
 - added theme copying into the container in templates
 - docs and reno updated

Change-Id: If9982c8e18be31772cb031ef72b7eebd4d768be5
Co-Authored-By: Jakub Darmach <jakub@stackhpc.com>
Depends-On: https://review.opendev.org/c/openstack/kayobe/+/824565
Depends-On: https://review.opendev.org/c/openstack/kolla/+/826672