OpenVPN: Add auth-user-pass to the client configuration

OpenVPN: Add auth-user-pass to the client configuration

Since we are doing a fake user authentication to get 2FA going, we need
to explicitley enable this. Usually clients were happy without this, but
somewhere it must have changed recently that clients require this set
explicitely.

Fixes: #13109 - openVPN, 2FA - client does not ask for One Time Token
Reported-by: Heino Gutschmidt <heino.gutschmidt@managedhosting.de>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>