[doc] Add 2 use cases to openstack_integration.rst

Use case #2 VMWare driver requires nova config validation
on startup.
Use case #3 Diagnostic API to query state of world for
OpenStack serivces, i.e. operating system configuraitons
and hardware.
Update service architecture scheme.
Update Rubick data model document with model classes diagram.

Change-Id: Ib1073762e53804474ba9410f78bf5ceeb3d538ba