Disable upgrade test if not configured

Change-Id: Idf171390675f54d313306f6ad19e21f51c9c685f

trove_tempest_plugin/config.py

@@ -91,7 +91,7 @@ DatabaseGroup = [
     ),
     cfg.DictOpt(
         'pre_upgrade_datastore_versions',
-        default={'mysql': '5.7.29'},
+        default={},
         help='The datastore versions used to create instances that need to be '
              'upgrade.',
     ),

trove_tempest_plugin/tests/scenario/base_actions.py

@@ -61,6 +61,8 @@ class TestInstanceActionsBase(trove_base.BaseTroveTest):
                     constants.DB_NAME)
 
     @decorators.idempotent_id("be6dd514-27d6-11ea-a56a-98f2b3cc23a0")
+    @testtools.skipUnless(CONF.database.pre_upgrade_datastore_versions,
+                          'Datastore upgrade is disabled.')
     def test_instance_upgrade(self):
         cur_version = self.instance['datastore']['version']
         cfg_versions = CONF.database.pre_upgrade_datastore_versions