[MRG] Fix GUI MPI available cores

[gtdang]

The GUI MPI was not working on HPC systems where the number of cores partitioned to an instance is less than the total cores of a node.

This fix uses the number of available cores to the process instead of the total number of cores of the system.

Work done

1. Use psutil to get number of available cores instead of total system cores for Linux and Windows. Mac will still use total system cores.
2. Exposed the number of cores selection for the MPI backend. Default is set to 1 and the maximum is set to the number of available cores.
3. Fixed the issue where the backend sub-options (n cores) was not displaying for the default (joblib) on startup

closes #870