Simplifying CeleryExecutor

@krishnap 

I removed the use of subprocess queues which took out most of the complexity. Also added a airflow subcommand "worker" which starts a worker.

TESTs: I've been running some backfills using SqlAlchemy as a broker and it seems to work just right.

I tried to scp from a node to another to see if this could be viable for logs, but ssh authentication isn't setup properly. Any idea how to set this up?