compute rpcapi 2.29 is not backwards compatible

patch 6fcf413 which added compute
rpcapi version 2.29 does not provide any backwards compatibility.

Fix this issue with 'self.can_send_version'

Fix bug 1217068

Change-Id: Idc49040b712058aab8d3ed769bdf18bdc0a4f067

nova/compute/rpcapi.py

@@ -648,17 +648,29 @@ def snapshot_instance(self, ctxt, instance, image_id, image_type,
                 topic=_compute_topic(self.topic, ctxt, None, instance))
 
     def start_instance(self, ctxt, instance):
+        if self.can_send_version('2.29'):
+            version = '2.29'
+        else:
+            version = '2.0'
+            instance = jsonutils.to_primitive(
+                    objects_base.obj_to_primitive(instance))
         self.cast(ctxt, self.make_msg('start_instance',
                 instance=instance),
                 topic=_compute_topic(self.topic, ctxt, None, instance),
-                version='2.29')
+                version=version)
 
     def stop_instance(self, ctxt, instance, do_cast=True):
+        if self.can_send_version('2.29'):
+            version = '2.29'
+        else:
+            version = '2.0'
+            instance = jsonutils.to_primitive(
+                    objects_base.obj_to_primitive(instance))
         rpc_method = self.cast if do_cast else self.call
         return rpc_method(ctxt, self.make_msg('stop_instance',
                 instance=instance),
                 topic=_compute_topic(self.topic, ctxt, None, instance),
-                version='2.29')
+                version=version)
 
     def suspend_instance(self, ctxt, instance):
         if self.can_send_version('2.33'):