Add an option to allow running multiple concurrent NH instances

When https://github.com/envoyproxy/nighthawk/pull/86 lands running multiple NH instances concurrently will be prohibited by default, as doing so may introduce measurement noise. However, there may be valid use cases for wanting to do so, and a careful test setup may eliminate the argument of introducing noise. Add an option like --instance-name=unique-name to allow running distinct multiple concurrent instances.