Write pipe list in config

Move this part of main.py

    pl = Pipeline()
    # pl.add_pipe(QueryExpansion(config))
    # pl.add_pipe(ChatNoirPipeLine(config))
    pl.add_pipe(LoadTopicPipe())
    pl.add_pipe(DocumentCleaningPipeline())
    # pl.add_pipe(SaveTopicPipe())
    pl.add_pipe(Remerging(config))

to the config.yaml and load the pipes dynamically by a list of names:

pipeline:
- "query_expansion"
- "chat_noir"
# ...
- "remerging"