Add ability to use Ansible groups in generated inventory. Fixes #2551.

This PR addresses issue #2551.  The following example Vagrantfile:

```ruby
# -*- mode: ruby -*-
# vi: set ft=ruby :

VAGRANTFILE_API_VERSION = "2"

Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
  config.vm.box = "ubuntu_saucy"
  config.vm.provision "ansible" do |ansible|
    ansible.playbook = "vagrant.yml"
    ansible.groups = {
      "group1" => ["default", "machinea"],
      "group2" => ["machineb"],
      "all_groups:children": ["group1", "group2"],
      "not_included:children" => ["group2", "group3"]
    }
  end
end
```

would produce the following inventory file:

```
# Generated by Vagrant

default ansible_ssh_host=127.0.0.1 ansible_ssh_port=2222

[group1]
default

[all_groups:children]
group1
```