Add Tag search to Python client.

[Olek Górajek]

And review Tag REST api to see if other things can be added.

[Olek Górajek]

And check if node exists before registering a new source node:

existing_node = self._get_node(node_name=node.name)
        if "name" in existing_node:
            raise DJNodeAlreadyExists(node_name=node.name)