cli: improve the CLI

• 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 (it now defaults to 'swh.journal.objects'),
• hide several options of the 'replay' command; these are now loaded preferably from the config file.