Ensure no "db" related functional/fullstack tests are skipped in the gate

Ensure no "db" related functional/fullstack tests are skipped in the gate

Currently neutron uses MySQLTestCase[1] and
PostgreSQLOpportunisticTestCase[2] for functional and fullstack tests
using a real MySQL/PostgreSQL database. These classes skip tests when
the db is unavailable (db not installed/configured, missing packages,
etc.) which is fine when tests are runned by developers but not when
runned by the gate jobs.

This change updates MySQLTestCase[1] and defines PostgreSQLTestCase[1]
as PostgreSQL oslo.db test class wrapper: when the db is unavailable,
these classes ensure tests will:

* fail in the gate (dsvm-functional, dsvm-fullstack jobs),
* be skipped by default otherwise (functional, fullstack jobs).

[1] neutron.tests.common.base
[2] oslo_db.sqlalchemy.test_base

Closes-Bug: #1404093
Change-Id: I77b12e728ce9a7b0222c3df081842635f6375a3e