Add reason parameter to RAPI client functions

Only the functions for starting, stopping and rebooting a VM had a reason
parameter. Now, all the RAPI client functions generating opcodes do.

Also, one test is expanded to verify that a RAPI request with both body and
query supports the reason parameter properly.

Fixes Issue 776.
Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Hrvoje Ribicic <riba@google.com>

test/py/ganeti.rapi.client_unittest.py

@@ -1271,13 +1271,15 @@ class GanetiRapiClientTests(testutils.GanetiTestCase):
     for mnh in [None, False, True]:
       self.rapi.AddResponse("14470")
       self.assertEqual(14470,
-        self.client.ModifyCluster(maintain_node_health=mnh))
+        self.client.ModifyCluster(maintain_node_health=mnh,
+                                  reason="PinkBunniesInvasion"))
       self.assertHandler(rlib2.R_2_cluster_modify)
       self.assertItems([])
       data = serializer.LoadJson(self.rapi.GetLastRequestData())
       self.assertEqual(len(data), 1)
       self.assertEqual(data["maintain_node_health"], mnh)
       self.assertEqual(self.rapi.CountPending(), 0)
+      self.assertQuery("reason", ["PinkBunniesInvasion"])
 
   def testRedistributeConfig(self):
     self.rapi.AddResponse("3364")