IAllocator: replace fake policy with real one

This small patch actually reads the allocation policy from the
IAllocator message.
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>

Ganeti/HTools/IAlloc.hs

@@ -104,8 +104,10 @@ parseGroup :: String              -- ^ The group UUID
            -> [(String, JSValue)] -- ^ The JSON object
            -> Result (String, Group.Group)
 parseGroup u a = do
-  name <- fromObj a "name"
-  return (u, Group.create name u AllocPreferred)
+  let extract x = tryFromObj ("invalid data for group '" ++ u ++ "'") a x
+  name <- extract "name"
+  apol <- extract "alloc_policy"
+  return (u, Group.create name u apol)
 
 -- | Top-level parser.
 parseData :: String         -- ^ The JSON message as received from Ganeti