This removes common.controller, common.extension, common.router, and
common.wsgi. Relevant code from common.wsgi (used by AuthContext) was
moved into keystone.server.flask.request_processing.middleware.auth_context.
keystone.api.discovery now uses keystone.flask.base_url
test_middleware and test_exception were modified to reflect the changes
to the remaining code from keystone.common.wsgi
keystone.common.authorization only holds a couple constants for auth
work now.
Routes is removed from requirements.txt
Release-Note for migration to flask added.
Change-Id: I81563b6a49c8f12ecade058a9483f3b6f070dc72
Closes-Bug: #1776504
184c84ae76