`run_vagrant` fix: provision upon 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 provision the vagrant box when
`run_vagrant` is executed.

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.