Fix unit tests failures, when psycopg2 is not installed
Fix unit tests failures, when psycopg2 is not installed

A few TestConnectionUtils test cases currently fail, when psycopg2 is
not installed, while they do not necessarily need to have the latter.

Mock parts of psycopg2 dialect in SQLAlchemy to make sure psycopg2
module is not imported in tests.

Closes-Bug: #1499216

Change-Id: I92c14f0e29a57e6e3ff4a8a8a1c25bbd16848d9a

openstack/oslo.db,openstack/oslo.db