bugfixes: variable assignments fix and milvus throws an error when text similarity is used

[ouonline]

1. wrong parameter names that are ignored by index.query()
2. set default query mode to 'embedding' if similarity is None
3. milvus does not support text queries
4. nodes should be created before store.query() is called when type is None or default
5. using add() instead of update() to insert single element into a set
6. set embed_keys so that it is assign to self._embed_keys properly in Retriever::__init__()