Run unit tests against PostgreSQL

* Creates a new tox env py-pgsql
* Adds a new script setupt-test-env-postgresql.sh to set up postgresql
* Renames the MySQLDbManager into SQLManage, used by both mysql and
  postgresql tests
* Creates a new scenario for posgresql in the
  MixinTestsWithBackendScenarios

Related to blueprint sql-unit-tests-on-real-backend
Co-Authored-By: Ala Rezmerita <ala.rezmerita@cloudwatt.com>

Change-Id: I60af82d5d8b5750884ba0d1ca726645f8b31448b
Merge "Run unit tests against PostgreSQL"