Remove auto-generation of db schema from models at startup

This patch removes the Neutron capability of creating database tables
from sqlalchemy models for all those model classes for which
a table is not found in the database schema.
Migrations should be the official and only solution for creating and
managing the Neutron db schema.
This patch also adapts unit tests in order to ensure test schemas
are still correctly created.

DocImpact
Update deployment documentation accordingly.

Closes-Bug: #1207402

Change-Id: Ie4ee5507888ecad5f6dc32ce7a029c43014687a2
Co-Authored-By: Henry Gessau <gessau@cisco.com>