core: fix calls to domish.Element.elements:

domish.Element.elements should be called with namespace and element name as 2 arguments,
but it has been confused in several places with the call to `domish.Element.addElement`
which is often done with a `(namespace, name)` tuple. Unfortunately calling with a tuple
is accepted and doesn't raise any error in `elements`, but this result in a wrong element
returned.

This patch fixes the erroneous calls.

sat/plugins/plugin_exp_invitation.py

@@ -268,7 +268,7 @@ class Invitation(object):
             args = [service, node, item_id, item_elt]
         else:
             try:
-                node_data_elt = next(pubsub_elt.elements((NS_INVITATION, "node_data")))
+                node_data_elt = next(pubsub_elt.elements(NS_INVITATION, "node_data"))
             except StopIteration:
                 raise exceptions.DataError("Bad invitation, ignoring")
             namespace = node_data_elt['namespace']