Allow provisioner plugins to register without config

Hello, while maintaining the [vagrant-hostmanager](https://github.com/smdahlen/vagrant-hostmanager) plugin, I noticed that vagrant 1.5 breaks when a provisioner plugin is registered without a provisioner config.

The init code in vm_provisioner.rb implies that it's okay not to have a config for the provisioner, but then it breaks in [#add_config](https://github.com/pbitty/vagrant/blob/10d5416a9037233a8610f9b16cb28c948fa1c533/plugins/kernel_v2/config/vm_provisioner.rb#L51) if the config class is nil.

I wrote a failing test for it and then added DummyConfig as the default config class.

Would this be the way to go in fixing it?  I'm happy to make any changes to the PR.