Add a "SimpleLauncher" for pre-bootstrapped images

Add a class that skips the apt/python/s3funnel dependencies for cases where
we use an image that already includes them. Use it for the "process incoming"
script.
Update launch_mapreduce_job to use SimpleLauncher

Remove a lot of duplicate code by extending the common Launcher.
Add the "bad record" message to the job output.
Start "process incoming" only on master