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
I did not take time to benchmark it, but yes at least with an "exhausted" developer workstation the latency time to start `vagrant` could be an annoying overhead (BTW, see https://github.com/mlafeldt/chef-runner#chef-runner---the-fastest-way-to-run-chef-cookbooks). It is one strength of ansible host-based provisioning to not have to update remote files to the target VM, so maybe it is not a bad idea to give some hints in this direction...

I'll fix the grammar mistake, but anyway, I'm quite open to update this part. I tried to give more reason to explain these "non-vagrant style" steps, but we also could consider removing this topic from the guide. More thoughts?