Ssh user

@sax, @kigster et. al - here's my fix to allow one to pass `--ssh-user`.  A few caveats:

* I haven't made this work for 'provisioner bootstrap', just 'provisioner provision'

* Passing `--ssh-user` doesn't work unless we also _don't_ pass `--distro` - @kevinykchan is looking at this a bit with me, but we don't have a fix yet.  Debugging boot(strap) stuff sucks :)

Improvements/feedback welcome of course.