Implementation of internal notification callbacks within Keystone

Keystone subsystems (extensions or other modules) can now
subscribe to Create, Update, and Delete events.  This allows any
subscribed system to act upon the event and perform actions (such
as cleanup on delete).

Co-Authored-By: Guang Yee <guang.yee@hp.com>
Co-Authored-By: Morgan Fainberg <m@metacloud.com>

bp: internal-callbacks
Change-Id: I03713b7f1f94480f76e0121eb6184226062af1a6
Merge "Implementation of internal notification callbacks within Keystone"