Rename, document, and provide defaults for extra core agent configuration variables

[adamchainz]

It's possible to set these config vars to change the way the core agent is launched:

• log_level
• log_file
• socket_path
• config_file

These should be prefixed with core_agent , like core_agent_log_level, and be documented. Would also be useful for the logging guide to point to setting core_agent_log_file to capture the logs - by default they go to stdout, which is nowhere, and it makes debugging harder.