Fixing get_panel_groups uncaught exception

The get_dashboards method in horizon/base.py returns dashboards
that have been unregistered. Subsequently calling get_panel_groups
on that dashboard causes an uncaught exception.

Fixes: bug#1185914

Change-Id: Ia5a7e7d19e7d2f04731a6840858cd9d9b91cfe00
Merge "Fixing get_panel_groups uncaught exception"