npm local package installation: added exec user param

required for cases, in which the user who runs the puppet agent is not the user who will actually use the local packages (e.g. a deamon/service based on local node packages is run by a technical user account, which is different from the user account used to run the puppet agent)
could do that if you can let me know, what exactly you mean by making this more descriptive ... code comments, variable names, etc.? to me it is already pretty descriptive.