Make event_sink publisher configurable

The charm currently configures events to be published to
rabbit on both the config.event_topic (event.sample queue)
and alarm topic but as of Queens Ceilometer no longer
consumes event.sample. This patch makes the event_sink
publishers configurable and defaults to publishing to
aodh to retain backwards compatibility.

Change-Id: I5b55f31adcf2b069ff51e387a416f9f1ac4099f8
Partial-Bug: #1676586
(cherry picked from commit d398cdff)