Update Ansible provisioner to only create a single inventory file

The Ansible provisioner will now only create a single inventory file named,
"vagrant_ansible_inventory". All defined Vagrant machines will be added to
this inventory file. Provisioning will now include a "--limit=#{machine}"
option to scope Ansible provisioning tasks to just the current machine. Setting
the Ansible provisioner's "limit" config option will override the new default
limit. Ansible provisioning scripts will now have access to all other defined
machines and what groups they reside in.

An example Vagrantfile and playbook to test these changes:

```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.box_url = "http://cloud-images.ubuntu.com/vagrant/saucy/current/saucy-server-cloudimg-amd64-vagrant-disk1.box"

  config.vm.define "machinea" do |machinea|
  end

  config.vm.define "machineb" do |machinea|
  end

  config.vm.provision "ansible" do |ansible|
    ansible.playbook = "vagrant.yml"
    ansible.verbose = "v"
    ansible.host_key_checking = false
    #ansible.limit = ""
    ansible.groups = {
      "foo_group" => ["machinea"],
      "bar_group" => ["machineb"],
      "all_groups:children" => ["foo_group", "bar_group"],
      "not_included:children" => ["sad_group"]
    }
  end
end
```

```
---
- hosts: foo_group
  tasks:
    - name: Testing foo
      command: echo foo

- hosts: bar_group
  tasks:
    - name: Testing bar
      command: echo bar
```