Switch away from using Nailgun DB session

Since we try to provide Nailgun with finalized response instead of
exceptions, it's DB session management doesn't know that session can be
invalidated already and tries to commit. We'll just use Nailgun's DB
connection string instead.

Change-Id: Id8f240c4dd75736015fabdff1333dff41741fb19

tuning_box/nailgun.py

@@ -15,7 +15,7 @@ from __future__ import absolute_import
 import itertools
 import threading
 
-from nailgun import db as nailgun_db
+from nailgun.db import sqlalchemy as nailgun_sa
 from nailgun import extensions
 import web
 
@@ -69,8 +69,8 @@ class TB2WebPy(App2WebPy):
     def create_app(self):
         app = tb_app.build_app()
         tb_db.prefix_tables(tb_db, Extension.table_prefix())
-        tb_db.db.session = nailgun_db.db
         app.config["PROPAGATE_EXCEPTIONS"] = True
+        app.config["SQLALCHEMY_DATABASE_URI"] = nailgun_sa.db_str
         return app