Updates to horizon settings.rst

Added documentation for auto_fade_alerts.

Also added to the OpenStack section documentation for:
OPENSTACK_API_VERSIONS,
OPENSTACK_KEYSTONE_MULTIDOMAIN_SUPPORT,
OPENSTACK_KEYSTONE_DEFAULT_DOMAIN,
OPENSTACK_KEYSTONE_BACKEND,
CONSOLE_TYPE,
TROVE_ADD_USER_PERMS,
TROVE_ADD_DATABASE_PERMS.

In the Django section, added ALLOWED_HOSTS
and a note about use of SECRET_KEY.

Closes-Bug: #1221115
Change-Id:  I16596fef2af62b892cef00ab25e2bfe4a80e1c65
Merge "Updates to horizon settings.rst"