BaseStorage: ensure that an alias node always has 'node_id' set when merging

The node has an id anyway but this is not necessarily set in the
dictionary used to merge the data.

ffstatus/basestorage.py

@@ -208,6 +208,7 @@ class BaseStorage(object):
                 continue
 
             # merge data
+            alias['node_id'] = alias_id
             update = self.__merge_alias_node(item, alias)
             updated[item_id] = update
             logging.debug("Merged alias '%s' into '%s'.", alias_id, item_id)