implement a release-engineering bus listener

[dparalen]

RH release engineering adopts a process similar to Fedmsg. Thus we need to implement an event listener for image-built announcement messages. The listener should trigger the validation command for incoming events and post result messages back on the bus. The format of the message is still under discussion. Also Fedora Cloud Community might want to adopt our test harness in case the listener supports the Fedimg messaging as well.