Name the shared broker in HispecDaemon and apply LIBBY_ environment overrides

[mikelangmayr]

• Set rabbitmq_url on HispecDaemon so the broker address is declared once instead of per daemon config; no deployed config sets it today
• Default from_config_file to env_prefix="LIBBY_", so LIBBY_<KEY> variables override config keys; it was passed as None, which read LIBBY_RABBITMQ_URL and silently ignored it
• The broker address carries no credentials, so a password can live in the root-only /etc/hispec/secrets.env rather than a config file every hispec-ops member can read
• Document the LIBBY_RABBITMQ_URL deploy step under Secrets
• Deploy step: add LIBBY_RABBITMQ_URL to /etc/hispec/secrets.env on each host before restarting daemons, since an off-host daemon cannot use guest and an on-host one no longer connects over loopback
• Verified on the camera host: with no credential the daemon is refused with ACCESS_REFUSED, and with LIBBY_RABBITMQ_URL alone it serves all 25 keywords with no broker line in its YAML
• pylint 10.00/10 on the changed module