Run jobs on Kubernetes

If the config includes a 'kubernetes' key, use it to configure a new job
runner implementation that replaces the Grid Engine one. (Though
admittedly the “choice” between the backends is mostly illusory – per
two commits ago, you wouldn’t really want to run this version of the
code on Grid Engine anyways. So the old runner will be removed soon
enough.)

I decided to create jobs via the Kubernetes API directly (via the Python
kubernetes library, which is decent), rather than using the Toolforge
Jobs framework; but I might still change that later. Having to manually
set the $TOOL_DATA_DIR and source the Apt build pack environment file
isn’t exactly ideal. It works for now, though.

Bug: T320140