Potential fix for #1207

I've changed the way `run_puppet_apply` computes the `--modulepath` value. 
Now, it should prepend the config modulepath to the system modulepath instead of clobbering it.

@mitchellh
This is my first commit to Vagrant. I couldn't find any tests for the `Puppet` class and am not sure if this is a good fix or not. Could you give me some suggestions on how I can verify whether this fixes the issue or not? And can I add an automated test somewhere?

Waiting to hear back. Happy to make further changes.