Create Data Sources panel in Horizon

Move existing Policies panel under a new Policy panel group. Add new Data
Sources panel, moving list of policy tables from the old Policy panel there.
Modify devstack script to update Horizon for running unit tests and README to
include Ceilometer services. Fix bug with wrapping Policy object twice in
congress.py:policy_get().

Implements: blueprint horizon-read-datasources
Change-Id: I3fe684d91d079ad1288f97b4e048c11b5edf986a