meter_dispatchers have to be set explicitly with mongodb backend in Ocata

[Nobuto Murata]

In Ocata cycle, meter_dispatchers was unset by default in favor of Gnocchi.
https://github.com/openstack/ceilometer/commit/8902dd10272c05ff3113f2b65d2c9c0f0540343b

While our preferred solution would be to use Gnocchi, etc. instead of MongoDB, we still needs to set meter_dispatchers=database for backward compatibility when mongodb relation is there. Otherwise, Ceilometer's meter with Mongodb stops working after an upgrade.

[Nobuto Murata (nobuto)]

https://review.openstack.org/#/c/495571/

[Nobuto Murata (nobuto)]

event_dispatchers seems not needed since it was removed in Newton already:
https://github.com/openstack/ceilometer/commit/3685dcf417543db0bb708b347e996d88385c8c5b

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/495571
Committed: https://git.openstack.org/cgit/openstack/charm-ceilometer/commit/?id=486b6f9718d86bfd156b0c39b9e2b318d788aab9
Submitter: Jenkins
Branch: master

commit 486b6f9718d86bfd156b0c39b9e2b318d788aab9
Author: Nobuto Murata <email address hidden>
Date: Mon Aug 21 00:33:59 2017 +0700

    Set event,meter_dispatchers=database explicitly when used with MongoDB

    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