Convergence Specification

The is the top-level spec and design for convergence.  There are
subsidiary specs:

(1) Convergence observer, at https://review.openstack.org/#/c/96394/
(2) Convergence engine, at https://review.openstack.org/#/c/96404/
(3) Convergence continuous observer, at
    https://review.openstack.org/#/c/100012/

Specifies bp convergence
Co-Authored-By: Mike Spreitzer <mspreitz@us.ibm.com>
Change-Id: I51f071e8fa93171395021e5f78ac8f4861e1ef62
Merge "Convergence Specification"