Starting an experiment with customized settings

[liehe]

Is your feature request related to a problem? Please describe.
For the moment, the exec command for mpi jobs is hard-coded in mlbench/master/api/utils/run_utils.py. This, however, limits the choice of argument passed to the client script. On the worker side, config files are used so that we don't have to change the code on the master.

Describe the solution you'd like
The main.py on the workers only uses line arguments not config files. Users can upload a file of program arguments or write the arguments in an input box.