Add support for remote consumers of Ceilometer event data.

There is a use-case to forward events received by ceilometer from
OpenStack services to an external system via http/udp. It comes
from the fact that aodh does not include all information about an
event alarm into POSTed data. Raw event data can be published by
ceilometer via http/udp to an external system without giving a
user direct access to rabbitmq.

This patch adds a context generator, associated templates, and
configuration entry for configuring multiple event-sinks in
/etc/ceilometer/event_pipeline.yaml. This also modifies the
ceilometer.conf templates to reflect the correct naming of the
same file to align with upstream Ceilometer.

Also adds support for a future Panko charm event consumer using
the event-service interface in the above context with associated
metadata and hook symlinks.

This change will only effect Mitaka and later clouds.

Closes Bug: 1763321

Change-Id: I931438c720272bd9a3d2b958ebabcd3584790bd0