Log message header in MPI runs

[simoneriggi]

Log message has this format:

date [hostname, PROCXXX, TIDXXX] INFO My log message

Where PROC is the MPI processor rank and TID is the thread id in OpenMP. In MPI+OpenMP runs the header info [hostname,PROC,TID] cannot be retrieved and are empty string for computing nodes different from the frontend.
To be fixed.