Support vagrant-softlayer plugin syntax

Enable support for [vagrant-softlayer](https://github.com/audiolize/vagrant-softlayer)'s Vagrantfile syntax:

`.kitchen.yml`:
```
---
driver:
  name: vagrant
  provider: softlayer
  customize:
    username: <%= ENV['SL_API_USERNAME'] %>
    api_key:  <%= ENV['SL_API_KEY'] %>
    #...
```

Configuration generated:
```
Vagrant.configure("2") do |config|
  config.vm.provider :softlayer do |sl|
    sl.username = '...'
    sl.api_key  = '...'
  end
end
```