Add notifications for Flavors

Project Searchlight have added plugin for nova Flavors:
https://blueprints.launchpad.net/searchlight/+spec/nova-flavor-plugin
user can use Searchligh to perform advanced search for nova flavors now,
but the data cannot be automatically updated as Searchligh listen to
notifications to update the related data and yet Nova didn't send out
any notifications when perform flavor related actions. If we can add
notifications it will be more convenient for both Searchlight and end
users.

This BP proposed to add notifications when:
- A new flavor is created
- A flavor has been deleted
- A flavor has been updated