Allow users to login without 2fa and removed pam dependency.

OpenVPN admins can now choose if they want to make 2fa mandatory. If
they're using pam, they can choose which users use 2fa and which don't
simply by keeping their logins out of the authy-vpn.conf file.

PAM is also no longer compiled as part of authy-openvpn as it's included
as part of openvpn anyway.