Update port conflict checker to distinguish between conflicts and exact matches.

This is a fix for #1359837 (https://bugs.launchpad.net/juju-core/+bug/1359837).

Opening a port that is already open should not result in an error.
Ah, sorry I see now there is 1 only, but I still think you can leave TCP in the test cases and inside the for loop check for combos like TCP,TCP; TCP,UDP; UDP,UDP; UDP, TCP.