feat: support LOG_CONFIG environment variable

This variable allows users to override the logging.conf used
for Python logging configuration. By default, this is set
to logging.conf, which is a production config. LOG_CONFIG
is treated relative to [options] aurwebdir.

This patch allows us to specify the logging config as opposed
to copying over logging.conf in our test docker and gitlab
test scripts, as well as ease-of-testing as a developer.

Signed-off-by: Kevin Morris <kevr@0cost.org>
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