Adds a setting custom-theme which operats similar to ubuntu-theme and
default-theme. The provided resource is placed in the themes folder and
apache is setup to serve static content for the theme. This leaves the
default theme untouched allowing the custom theme to override files
based on the built in horizon theme capabilities. For details on theming
capabilities see:
https://docs.openstack.org/horizon/latest/configuration/themes.html

gnuoy: retry logic for unrelated test updated after a number of CI
failures.

Closes-Bug: #1778284

Change-Id: I91ad19e8aad5c0e0773d42fa4f085cbcecb82458