Feature/export ssh auth sock

When using puppet/chef providers a sudo is issued from inside the vm. The thing is that if **SSH_AUTH_SOCK** is not set once in sudo, ssh-agent-forwarding will not work. This is due to the fact that sudo erases the environment unless set otherwise in sudoers file.

Given that there is a configuration variable to set agent forwarding, and that we can not make any asumptions of how **/etc/sudoers** is configured, it makes sense to correctly export that variable when we are using sudo and agent forwarding is set in the Vagrant file.