[autotest] We do not need to create cros-version label when provisioning

We add a cros-version label to a host when provision finishes.
We do not need to create the label in the provision special task.

BUG=chromium:506399
TEST=./cli/atest job create -d cros-version:nyan_big-release/R43-6872.0.0
     -b board:nyan_big --test=dummy_Pass dummy_Pass
DEPLOY=apache

Change-Id: Ie042bcdf6a35ade0ee3570743cd3a8a43d96140b
Reviewed-on: https://chromium-review.googlesource.com/283540
Tested-by: Mungyung Ryu <mkryu@google.com>
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Mungyung Ryu <mkryu@google.com>
diff --git a/frontend/afe/rpc_utils.py b/frontend/afe/rpc_utils.py
index 4b88bac..ac5d2ea 100644
--- a/frontend/afe/rpc_utils.py
+++ b/frontend/afe/rpc_utils.py
@@ -646,6 +646,8 @@
                              not because the label already existed.
     @returns True is a label was created, False otherwise.
     """
+    # Make sure this function is not called on shards but only on master.
+    assert not server_utils.is_shard()
     try:
         models.Label.objects.get(name=name)
     except models.Label.DoesNotExist: