Provision a test/playground topic for new users to read/write to

[deepchatterjeeligo]

Creating this issue at the request of @cnweaver.

Currently new users have access to the following topics

 __consumer_offsets
 amon.nuem
 gcn.circular
 gcn.notice
 sys.heartbeat

It would be convenient to expose some kind of a testing topic with write access. This can be lower in terms of priority of message delivery, rate limited if need be, and for the purpose of helping newcomers to design their workflows. It is to be understood that this is for the sole purpose of testing and that anyone can be writing to this topics, so there is no guarantee of type or sequence of message received. Also that delivery may be delayed in case there is excessive testing traffic.