Allow event filter based on an existing user

If you specify the following in rabbitmq.config:

[security]
    authUser = jenkins_verifier

It will filter out all events that is not visible for
that Gerrit-user. In this case "jenkins_verifier".

Change-Id: I28411ca3795badcbf9ff5fe95039e8f84c590055
Fix conflict
Merge branch 'GLundh-authuser-support'