Change Supervisor runtime, move Supervisor socket.

Configuration now reads a symlink at /run/supervisor/$HOSTNAME. This
allows Supervisor to use the %(here)s setting so that we can trick it
into puting the socket and pid files into a dynamic directory at
/run/supervisor/$HOSTNAME.

This sets us up later for exposing all the wheels sockets via the /run
directory in the hub for admin processes and monitoring.

We also add a file at /run/supervisor/$HOSTNAME/$SPOKE_NAME so that we
can easily see which spoke those socket/pid files are for. Seems easier
then having the admin/monitoring process derive it via a docker daemon
lookup or some other potentially faulty guessing method from log files
or something.