Set default value for `type` to `:host` when it's nil

I got an error as below:


```
/Users/kentaro/.rbenv/versions/2.0.0-p0/lib/ruby/gems/2.0.0/gems/vagrant-1.1.0.dev/plugins/provisioners/chef/provisioner/chef_solo.rb:81:in `expand_path': no implicit conversion of nil into String (TypeError)
```

It occurs when users set `roles_path` like `chef.roles_path = "path/to/roles"`.

[This change](https://github.com/mitchellh/vagrant/commit/1af32555595a8c762b794e053d53441216e8085d#L1R65) assumes `type` is always set, but it can be nil.