Fix dashboard login errors

Set WSGI process to run as horizon user

Set ownership on secret_key file so it can can be read by dashboard

Make ALLOWED_HOSTS configurable in local_settings.py for use in
production.

Added test for ALLOWED_HOSTS

Change-Id: Ide506511c3dcc075f181e47b2147af2d6107acf7
Closes-Bug: #1267691