Code review comment for lp:~canonical-ci-engineering/uci-engine/doc-msging

I'm working on an update to this. So as not to delay, here is the summary:

There is no need to define a workflow up front. So the snappy image watcher need not be concerned with this. Instead each service will send its output message to a default queue. For the snappy image watcher, we can begin by calling this 'snappy.images'. So the watcher would output a message on this queue that has the information on the image to test and the test branch to use (the one with the snappy dep8 tests). It may need to add other data, but we'll have to figure that out later

The snappy image watcher and every other service will be deployed to send their output messages to a default queue. However, to facilitate writing tests, the output queue can be changed by specifying a specific queue in the received message via the 'output_queue' key. This doesn't directly apply to the snappy image watcher as it has no input queue. However, it make sense to have some similar mechanism to override the output queue for writing tests (will try to put some thought into this)