Ports activation: fix issues in cluster layer

* Don't require presence of 'active' node, to avoid breakage with
  ports created prior to the the existence of the 'active' attribute,
  when ports sets were the mechamisn to track port liveness.
* Don't trigger getConfig() calls when port liveness changes as that
  leaks the watcher set up by the previous getConfig() call.

Change-Id: I3d85e380211ecd7b30329426165617c1370666aa
Signed-off-by: Guillermo Ontanon <guillermo@midokura.jp>