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.
I don't see how this makes the code easier to read. I got rid of the ErrorPortsMatch error, so now CheckConflicts returns an error only when port ranges conflict (but do not match exactly).