Ability to generate coherent ansible inventory file (ansible.inventory_file parameter is now optional)

Advantages to use auto-generated Ansible inventory file:
 * more DRY configuration files between Vagrant and Ansible settings
 * ensure that vagrant will only provision VMs managed by vagrant itself
 * works fine in multi-machine mode (each provision step can only connect to related VM)

Note: So far, I opened 3 separated pull requests (see #1697 and #1699). Let me know if you prefer to treat them together in a single pull request.