Disable automatic config file discovery for pep8 target
When trying to run pep8 target on a system with installed
/etc/neutron/neutron.conf file, neutron-db-manage may fail due to
access denied to the configuration file. This is because oslo.config
attempts to discover configuration files automatically if they are
not passed with --config-file argument.
To disable auto discovery, we pass --config-file to the tool.
(cherry picked from commit