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:

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##### Actual Results:

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OK, good to known. Will do :)