Complete runtime type coercion. Closes #443.

Used in lieu of [runtime] validation in config.py, and in the broadcast
command to coerce command line string values to the right type. This
also corrects a bug in the broadcast command: the try block must be
inside the coercion loops or it aborts at the first KeyError.
Adjust for type-coerced timeout values.
Merge pull request #444 from hjoliver/443.runtime-type-coercion

Complete [runtime] type coercion. Closes #443.
Keep the ssh tunnel alive while background tasks run.
Merge pull request #445 from hjoliver/non-detaching-background-jobs

Keep the ssh tunnel alive while background tasks run.