Add project CRUD to assignment_api Manager

Specifically, the "proxy" methods on identity_api for following methods
are removed in favor of assignment_api:

- create_project
- update_project
- delete_project

The majority of this change is a refactor in support of the incomplete
work done for bp split-identity in
https://review.openstack.org/#/c/33745/

Notification decorators are also added to these 3 methods in support of
bp notifications and bug 967832.

Change-Id: I81b4eca720615b38a38858aecd734af8ab383d91
Merge "Add project CRUD to assignment_api Manager"