Create a Collector which can store raw Events.

This introduces a new service called ceilometer-collector-event
which will grab notifications and store them in the Event and
Trait tables. The only driver currently implemented is for
AMQP via Kombu and librabbitmq.

This is different than the existing notification collector in that
it will only ack() the message when the event is successfully
persisted and it doesn't need to work around the oslo.rpc "method"/
"parameters" structure. It reads the notifications as they were sent.

More will need to be done to this to extract all the popular
traits from the events.

Change-Id: I9723134d6647ce2f7a90e2bc8633c6951bc9d8cf