Documentation: Update the Vagrant Guide

##### Issue Type:

Docs Pull Request

##### Ansible Version:

* devel
* v1.8.2

##### Environment:

N/A

##### Summary:

* Solve problems raised in #7665 by @maspwr
* Mention that this guide corresponds to Vagrant 1.7 features and for now invite to rely on `config.ssh.insert_key = false` to avoid private key mess in a multi-machine environment.
* Remove the `vagrant init` example, which is not really useful when following this guide
* Invite to gather advanced details direclty in the Vagrant Ansible Provisioner documentation, to avoid repetitions/discrepancies over time.
* Explain how to know the exact `ansible-playbook` command that is generated and used by the Vagrant Ansible provisioner.

##### Steps To Reproduce:

N/A

##### Expected Results:

N/A

##### Actual Results:

N/A
The latest version of Vagrant doesn't create the `VAGRANTFILE_API_VERSION` variable anymore. It just uses `2` here directly.

suggestion: Delete lines 36, 37, & 38 above and replace `VAGRANTFILE_API_VERSION` with `2` on this line.