Use more specific asserts in tests

Instead of assertTrue and assertFalse use more specific asserts.
They are compatible with Python 2.7[1] and 3.4[2]

[1]: https://docs.python.org/2.7/library/unittest.html
[2]: https://docs.python.org/3.4/library/unittest.html

Change-Id: I3ac49863641e26841210d020e8bca0bd93176423
diff --git a/tempest/api/compute/admin/test_agents.py b/tempest/api/compute/admin/test_agents.py
index 61359f1..40cb523 100644
--- a/tempest/api/compute/admin/test_agents.py
+++ b/tempest/api/compute/admin/test_agents.py
@@ -90,7 +90,8 @@
         body = self.client.create_agent(**self.params_agent)['agent']
         self.addCleanup(self.client.delete_agent, body['agent_id'])
         agents = self.client.list_agents()['agents']
-        self.assertTrue(len(agents) > 0, 'Cannot get any agents.(%s)' % agents)
+        self.assertGreater(len(agents), 0,
+                           'Cannot get any agents.(%s)' % agents)
         self.assertIn(body['agent_id'], map(lambda x: x['agent_id'], agents))
 
     @test.idempotent_id('eabadde4-3cd7-4ec4-a4b5-5a936d2d4408')
@@ -108,7 +109,8 @@
         agent_id_xen = agent_xen['agent_id']
         agents = (self.client.list_agents(hypervisor=agent_xen['hypervisor'])
                   ['agents'])
-        self.assertTrue(len(agents) > 0, 'Cannot get any agents.(%s)' % agents)
+        self.assertGreater(len(agents), 0,
+                           'Cannot get any agents.(%s)' % agents)
         self.assertIn(agent_id_xen, map(lambda x: x['agent_id'], agents))
         self.assertNotIn(body['agent_id'], map(lambda x: x['agent_id'],
                                                agents))