scripts: policy-node: filter-like streams should not follow default

Filter-like streams (e.g. output stream from filter-chain) should not start following default sink, as usually the user intends their routing to remain static, and not change unless explicitly requested. Pulseaudio has special handling for streams associated with filter sinks/sources, and does not make such streams follow default.

Use presence of node.link-group to determine which streams are associated with filter sinks/sources, and don't make them to follow default.

See #231 (closed)