Updated default Vagrantfile template

I need to generate following Vagrant config:
```ruby
  c.ssh.private_key_path = ["~/.ssh/id_rsa", "~/.vagrant.d/insecure_private_key"]
  c.ssh.forward_agent = true
```
This is required if I want to pull git repository on the VM, using keys from the host. Vagrant allows setting  array of private keys path (see [comit](https://github.com/mitchellh/vagrant/commit/45e09eb6778fd22d7d6cabae65f476d9f8035873)). Therefore kitchen-vagrant should also support without having to use custom Vagrantfile template.

I've also added ```:ssh_forward_agent``` option which maps to ```config.ssh.forward_agent`` in Vagrant, which seems to be used very often with ```private_key_path``` setting.

With this change user will be able to setup driver like this:
```yml
driver:
  name: vagrant
  ssh_agent: true
  ssh_key: ['~/.ssh/id_rsa', '~/.vagrant.d/insecure_private_key']
  ..
```

What do you think?