Explicitly set ConfigOpts._args to sys.argv[1:]

Fixes bug #1146426

If you don't pass an args list to ConfigOpts.__call__() we pass
args=None down to ArgumentParser.parse_args() and it defaults to
sys.argv[1:].

However, in log_opt_values() we reference sys._args and you'd expect it
to show sys.argv[1:] in this case too but it shows None.

Fix this by explicitly setting _conf to sys.argv[1:] rather than relying
on argparse's behaviour.

Also add a test which would fail without this fix.

Change-Id: Id5a769bdda5f2359da18c301cb6305aaf3ed8617