Allow option to specify just 1 of user and pass in OpenVPN .up file

As per comment in https://redmine.pfsense.org/issues/3633 sometimes the server end only requires a password, no username. Usually 1 long string that serves as the hard-to-guess authentication. OpenVPN expects something to be on the first line of the ".up" file - traditionally called the username. It also insists on the second line being present, but is happy with it being empty - this is the authentication information traditionally called "password".
Let the user put the single piece of authentication information in either the Username or Password field on the web GUI - whichever they feel comfortable calling it. In the ".up" file it has to always be the first line to keep OpenVPN happy.