Added isSubscribed() method to instance notifier event message receiver
Preventing topology event message delegater thread hang if an exception is thrown.

If an excetion occured, exception is logged and continue work.
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/incubator-stratos
Add a warning if the received event is not InstanceStartedEvent or InstanceActivatedEvent
Merge remote-tracking branch 'origin/master'