[Bugfix] Update log_level parsing to use parse_level from #27

Currently `StandardSupervisor` class method `_setup_logger` has the same issue fixed by #26 #27 where it cannot handle case where log level env variable is specified as an int (read as string).

We want to update this so it can handle this case as well as silently fallback to a specific level (e.g. ERROR) if parse fails.

https://github.com/aws/model-hosting-container-standards/blob/3a1411671caad8580fe214f990db3f1ae0237f5c/python/model_hosting_container_standards/supervisor/scripts/standard_supervisor.py#L116-L119

See `parse_level`:

https://github.com/aws/model-hosting-container-standards/blob/3a1411671caad8580fe214f990db3f1ae0237f5c/python/model_hosting_container_standards/logging_config.py#L9-L25

Start in python/model_hosting_container_standards/supervisor/scripts/standard_supervisor.py at StandardSupervisor._setup_logger, then read parse_level in python/model_hosting_container_standards/logging_config.py. Update the log-level handling so integer values supplied through the environment work and invalid values silently fall back to a specific level such as ERROR. Confirm the logger uses the parsed or fallback level.

Written by the indexing model from the issue text.