Change `run_vagrant` to copyfiles for each run and use `vagrant ssh-config`

Vagrant 1.3+ no longer automatically provisions upon `vagrant up` unless it is the first time that `vagrant up` is executed (see mitchellh/vagrant/issues/2423). Because tests are run after changes are made to the dokku code, we explicitly run `make copyfiles` to refresh the vagrant box's dokku install at the beginning of the test run. A note is added to remind the user to destroy or reprovision the vagrant box if dokku needs to be re-installed from scratch.

Existing ssh configuration may fail if user assigns dokku.me to 10.0.0.2 in hosts file (as recommended by `Vagrantfile`), since vagrant's SSH must go through 127.0.0.1. Thus, ssh configuration is now set with the `vagrant ssh-config` command, which maps dokku.me to 127.0.0.1 (and correct port) for SSH.