Hello MPI switch in CUD!

- Discovery of task launching at agent startup now creates a set of available
  methods.
- Pass mpi flag from CUD all the way to the ExecWorker where the translation to
  a specific launch method is made.
- Removed task_launching_method from: pilot_launcher_worker, bootstrapper and
  agent.
- Removed launch_task_method from all resource configs.
- Added mpi flag to india example.