Update reset_db to call setup if _DB is None.

This resolves issues when running individual tests that require
a database. With this fix I can once again run individual test
modules that make use of the database. For example:

  ./run_tests.sh test_libvirt

Previously this would fail with database errors.

Fixes LP Bug #1032738.

Change-Id: Icce7ac9414f0e19eece44819a217634947de7f73

nova/tests/__init__.py

@@ -61,6 +61,8 @@ def reset_db():
         conn = engine.connect()
         if _DB:
             conn.connection.executescript(_DB)
+        else:
+            setup()
     else:
         shutil.copyfile(os.path.join(FLAGS.state_path, FLAGS.sqlite_clean_db),
                         os.path.join(FLAGS.state_path, FLAGS.sqlite_db))