Ansible Provisioner: Support SSH-Forwarding and Multiple SSH Private Keys 

These two Vagrant options were so far not supported by the Ansible provisioner. As already mentioned, these settings only can be supported when running `ansible-playbook` in `ssh` communication mode, but not in `paramiko` mode.

I'd like to discuss whether `--connection=ssh` should be systematically forced, or not (see the third commit: 51e7bd9).

> Paramiko is the default SSH connection implementation on Enterprise Linux 6 or earlier, and is not used by default on other platforms.

See:
* http://docs.ansible.com/intro_configuration.html#paramiko-specific-settings
* http://docs.ansible.com/intro_getting_started.html#remote-connection-information

So, we could drop 51e7bd9 and make it clear in the Vagrant documentation, that Ansible Provisioner always enables `ssh` mode by default (`ansible.raw_arguments` can be used to force `paramiko` mode, when running Vagrant on a RHEL host for instance).

I'm quite OK with both variants, and you @mitchellh what do you think? I'll update the docs (and maybe the commits) according to your preference...

/cc @picsolvebryan Thanks again for reporting the SSH Forwarding bug fix. I assigned you the related commit and hope it's ok for you so :)