pptp: add 'pptp_passwd_file' test
NM can read passwords for VPNs from file. There was a bug in 1.12 and we were only able to read vpn.secret but vpn.secrets was supported before. We now support both versions to preserve backward compatibility.
https://bugzilla.redhat.com/show_bug.cgi?id=1628946
@Build:nm-1-12