Read the broker credentials from the environment instead of the URL

[mikelangmayr]

• HispecDaemon composes the broker URL from HISPEC_RABBITMQ_USER and HISPEC_RABBITMQ_PASSWORD, so the hostname stays in git and only the credentials live in /etc/hispec/secrets.env
• Supplying credentials previously meant setting LIBBY_RABBITMQ_URL, which replaces the whole URL and so duplicated the hostname on every host
• LIBBY_RABBITMQ_URL still works and still wins, for pointing one host at a different broker
• Credentials are percent-encoded, so a password containing @ or / cannot break the URL apart
• Docs updated, including an example that still showed the old DHCP address