The default value of event and meter_dispatchers were "database"
implicitly. However with Newton, there is no default value for
event_dispatchers. And empty meter_dispathers with Ocata. While Gnocchi
and Panko are preferred solutions, keep backward compatibility to
support an upgrade for existing deployments with MongoDB.

event_dispathers is not needed for Ocata since v2/events API has been
removed completely from Ceilometer.

Change-Id: I3a0f3fb029136d106afe0e77b704ee0c303ab27f
Closes-Bug: #1711932