Bug #694880: nova-compute now depends upon Cheetah even when not using libvirt

Only import Cheetah when needed, as we do already with libvirt and libxml2.
This ensures that users of other virt backends don't need Cheetah to run
nova-compute.

nova/tests/test_virt.py

@@ -33,6 +33,7 @@ flags.DECLARE('instances_path', 'nova.compute.manager')
 class LibvirtConnTestCase(test.TestCase):
     def setUp(self):
         super(LibvirtConnTestCase, self).setUp()
+        libvirt_conn._late_load_cheetah()
         self.flags(fake_call=True)
         self.manager = manager.AuthManager()
         self.user = self.manager.create_user('fake', 'fake', 'fake',