Add support to more than one db provider for tests

As db api worked with one EngineFacade singleton, the migrations tests
could not run successfully to MySQL and PostgreSQL database. The
migrations started being applied to the MySQL database (and EngineFacade
was instanciated based on it) and after to PostgresSQL, but EngineFacade was
not instantiated anymore, keeping with the MySQL engine.

In order to fix that issue, this commit implement a way so that the db api
works with one EngineFacade by database vendor.

Co-Authored-By: Moises Trovo <mtrovo@thoughtworks.com>

Change-Id: I07898bd963c08e7a2eff946b8f0c0886f2143474

subunit2sql/db/api.py

@@ -18,6 +18,7 @@ from oslo_config import cfg
 from oslo_db.sqlalchemy import session as db_session
 from oslo_db.sqlalchemy import utils as db_utils
 import sqlalchemy
+from sqlalchemy.engine.url import make_url
 
 from subunit2sql.db import models
 from subunit2sql import exceptions
@@ -27,16 +28,20 @@ CONF = cfg.CONF
 
 DAY_SECONDS = 60 * 60 * 24
 
-_FACADE = None
+_facades = {}
 
 
 def _create_facade_lazily():
-    global _FACADE
-    if _FACADE is None:
-        _FACADE = db_session.EngineFacade(
+    global _facades
+    db_url = make_url(CONF.database.connection)
+    db_backend = db_url.get_backend_name()
+    facade = _facades.get(db_backend)
+    if facade is None:
+        facade = db_session.EngineFacade(
             CONF.database.connection,
             **dict(CONF.database.iteritems()))
-    return _FACADE
+        _facades[db_backend] = facade
+    return facade
 
 
 def get_session(autocommit=True, expire_on_commit=False):