[Vagrant guide] Updating private key location

Since Vagrant 1.7.0, released in December 2014, the private key file is autogenerated and has a new location - check the changelog for more info: [version 1.7.0 changelog](https://github.com/mitchellh/vagrant/blob/master/CHANGELOG.md#170-december-9-2014)

docsite/rst/guide_vagrant.rst

@@ -107,14 +107,16 @@ inventory file may look something like this:
 
 If you want to run Ansible manually, you will want to make sure to pass
 ``ansible`` or ``ansible-playbook`` commands the correct arguments for the
-username (usually ``vagrant``) and the SSH key (usually
-``~/.vagrant.d/insecure_private_key``), and the autogenerated inventory file.
+username (usually ``vagrant``) and the SSH key (since Vagrant 1.7.0, this will be something like
+``.vagrant/machines/[machine name]/[provider]/private_key``), and the autogenerated inventory file.
 
 Here is an example:
 
 .. code-block:: bash
    
-    $ ansible-playbook -i .vagrant/provisioners/ansible/inventory/vagrant_ansible_inventory --private-key=~/.vagrant.d/insecure_private_key -u vagrant playbook.yml
+    $ ansible-playbook -i .vagrant/provisioners/ansible/inventory/vagrant_ansible_inventory --private-key=.vagrant/machines/default/virtualbox/private_key -u vagrant playbook.yml
+
+Note: Vagrant versions prior to 1.7.0 will use the private key located at ``~/.vagrant.d/insecure_private_key.``
 
 .. seealso::