Sync notifier module from Oslo

Pull in the common notifier from Oslo-incubator into Keystone. This only
introduces the notifier module and it's dependencies. This change is
standalone and doesn't contain any code to implementation notifications,
just the initial sync.

blueprint notifications

Change-Id: If62d6012a92e944f3196dd20e6cdd3236e7ecae2
Merge "Sync notifier module from Oslo"