This also removes the "Running (command)..." output since users will
expect to be able to redirect the ssh-config output to a file without
that noise.
If we really like the "Running" thing we can try and swing around and
get fancy by detecting whether or not we have a terminal or a file as
output and doing conditional stuff, but this seemed like the simplest
way forward for now.
Still work left to do on exec and stderr separation but this represents
enough working to let vagrant-spec assertions pass so I think it's worth a
checkpoint!