Read Linux package config from environment variables

Changed

• Read from environment variables for config in standalone agent. When the agent is started with the --config CLI option, it will now also read from the environment variables documented in the standalone agent config section. The environment variables are read first. Any option set in the config file (specified with --config) will override the environment variable value.
• Improve the error message that is logged for the standalone agent using a configuration while when it is not properly configured.

This release can be installed through our standalone agent packages and as a Docker image.