enabled runners to use the run_cmd option

Hi @mortenvp, @petya2164. I've re-enabled the run_cmd option for the external runners. This way modifications of the the command to be run is easy-peasy. This way users are able to add special command line arguments to, e.g., benchmarks, run by the runners.

I'm just gonna merge the pull request pretty fast, as I can only test whether it works on all the slaves by having the changes in the master. :/