Fix Exceptions doc page generation

The problem caused by circular import of django.contrib.auth
by both horizon.exceptions and openstack_auth.utils
(through openstack_dashboard.settings)

Change-Id: I7ad178f5dd7f43bf95a3b2091113725c3415533f
Closes-Bug: #1276243
Plugin-based panel group configuration

Panels can be added through plugin configuration, but it is broken
if the user can only use existing panel groups. This adds the
capability to create an empty panel group that can be used by panel
configuration to add itself to the group.

Change-Id: I28d6072fc05b3a4ef5a41cd68337a7864c560525
Closes-Bug: #1287981
Merge "Fix Exceptions doc page generation"
Merge "Plugin-based panel group configuration"