In the process of creating an instance, Nova will emit an
instance.update notification with 'building' state.
This will cause a KeyError exception because this instance
isn't in Watcher datamodel.
So we should ignore the notification instance.update with
'building' state.
Closes-Bug: #1832154
Change-Id: I950eec50d2cee38bd22c47a70ae6f88bbf049080