Blacklist workers

Add endpoint for blacklisting workers - Overlord will not assign new tasks to blacklisted workers, 
can be used to do rolling restarts of the worker nodes during release upgrades. 
This won't be persisted and can't be bootstrapped from mysql or ZK, so if the overlord changes leadership during this period for some reason, the worker will be un-blacklisted. That could throw a wrench in a currently progressing rolling restart.

I think that could be fixed by persisting the blacklist of workers to mysql or ZK, or by not having a blacklist, but actually just having an API on each worker that sets its version to the empty string. As long as the default minWorkerVersion in the RTR (and default worker version on the middle managers) is something else, that will cause the RTR to stop assigning tasks to it. Even if the overlord restarts.

I prefer the second method 'cause it uses a system we already have.