tests: allow to skip if no database URL

This is needed for certain driver that we may not want to test if
there's no URL provided in the scenario, i.e. no environment variable
can be found.

Change-Id: I0301987885184ce1e2f83c10db99dba70450449a

ceilometer/tests/db.py

@@ -33,6 +33,10 @@ from ceilometer.tests import base as test_base
 class TestBase(test_base.BaseTestCase):
     def setUp(self):
         super(TestBase, self).setUp()
+
+        if self.database_connection is None:
+            self.skipTest("No connection URL set")
+
         self.CONF = self.useFixture(config.Config()).conf
         self.CONF.set_override('connection', str(self.database_connection),
                                group='database')