also set the main group name to 'journal' so the main 'swh' cli tool
can generate shell completion properly.
- add more doc strings to main cli commands,
- adapt the main cli group to be used as a subcommand of the swh.core's main 'swh' command,
- fix handling of the configuration file loading mechanism,
- rename the 'consumer_id' argument as group_id, since this is indeed the group_id argument used to instantiate the underlying KafkaConsumer,
- make the 'prefix' argument optional and default to the global variable swh.journal.DEFAULT_PREFIX = 'swh.journal.objects'.
- hide several options of the 'replay' command ; these are now loaded preferably from the config file.