Run remora without executable

[carlosrosales]

To monitor system, or to monitor complex workflows that require remora running on every node independently of how the individual workflow pieces are launched within a session.

Two simple ways to do this are to:

1. watch for an environmental variable (REMORA_EXIT) until it is set (or set to 1)
2. watch for the existance of a file (remora.exit)

I think 1) works better and with less overhead. We can add this by giving remora a command line option, a environmental varialbe check (if REMORA_EXIT is set to 0, for example) or by creating an specilaized launch script.