Update watch_ports

ServiceUnitState.watch_ports should be modified such that it follows what was done for ServiceState.watch_exposed_flag (#788825), namely that the callback management should use the same code path (testing client.connected upon callback entry) and it should be made stateful and reliable.

This can cause a failure in ensemble.agents.tests.test_provision.ExposeTest.test_service_exposed_flag_changes when repeatedly looped and this patch is made to the test suite: http://paste.ubuntu.com/633766/