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.
This error message seems a bit too repetitive, I'd trim it like this "mismatched port ranges: 100-200/tcp (wordpress/0) and 100-300/tcp (wordpress/0) conflict", but this is just a suggestion, I can live with it.