Avoid using assertGreater

This doesn't exist in python 2.6.

Change-Id: I8d41a568ac4c9fc7330077e6643b40eddc071f91
diff --git a/tempest/api/compute/servers/test_server_actions.py b/tempest/api/compute/servers/test_server_actions.py
index e4af2e1..1822701 100644
--- a/tempest/api/compute/servers/test_server_actions.py
+++ b/tempest/api/compute/servers/test_server_actions.py
@@ -86,7 +86,8 @@
             # Log in and verify the boot time has changed
             linux_client = RemoteClient(server, self.ssh_user, self.password)
             new_boot_time = linux_client.get_boot_time()
-            self.assertGreater(new_boot_time, boot_time)
+            self.assertTrue(new_boot_time > boot_time,
+                            '%s > %s' % (new_boot_time, boot_time))
 
     @skip_because(bug="1014647")
     @attr(type='smoke')
@@ -106,7 +107,8 @@
             # Log in and verify the boot time has changed
             linux_client = RemoteClient(server, self.ssh_user, self.password)
             new_boot_time = linux_client.get_boot_time()
-            self.assertGreater(new_boot_time, boot_time)
+            self.assertTrue(new_boot_time > boot_time,
+                            '%s > %s' % (new_boot_time, boot_time))
 
     @attr(type='smoke')
     def test_rebuild_server(self):