Reintroduce ONIONBALANCE_LOG_LOCATION environment variable

Description

While the shipped init scripts still supports the ONIONBALANCE_LOG_LOCATION environment variable, the underlying codebase doesn't.

This was present in the old v2 tree:

onionbalance/onionbalance/hs_v2/config.py:LOG_LOCATION = os.environ.get('ONIONBALANCE_LOG_LOCATION')
onionbalance/onionbalance/hs_v2/manager.py:    if config.LOG_LOCATION:
onionbalance/onionbalance/hs_v2/manager.py:        log.setup_file_logger(config.LOG_LOCATION)

Tasks

• Reintroduce the ONIONBALANCE_LOG_LOCATION environment variable.
• Test.
• Update the documentation.
• Update the manpage.