state: openedPortsWatcher no longer exposes global keys

Follow-up to #799: the openedPortsWatcher in state now
reports changes as "\<machine-id\>:\<network-name\>", rather
than as global keys ("m#\<id\>#n#\<name\>"), as this exposes
internal state details unnecessarily.

NOTE: Already approved as https://github.com/dimitern/juju/pull/1
and http://reviews.vapour.ws/r/85/, just re-proposing against juju:master.