add new options for reassign partition to better manager dead brokers

Four changes here each requiring a) system tests b) unit test c) code to-do the actual work and 4) we should run it on dexter too (we should post the patch before running in test lab so others can do the same too at that time).

--replace-broker
--decommission-broker

fix two bugs
1) do not allow the user to start reassignment for a topic that doesn't exist
2) do not allow reassign to brokers that don't exist.

There could be other reassign like issue that come up also from others. My initial preference is one patch depending on what the issues/changes are and where in the code we are too.