Make pep8 job succeed when /etc/neutron/neutron.conf is not installed

Make pep8 job succeed when /etc/neutron/neutron.conf is not installed

Currently, if /etc/neutron/neutron.conf is not installed in the system,
neutron-db-manage fails in oslo.config code when trying to determine the
default configuration file to use.

Test job should not rely on any contents inside /etc/.

Instead, pass --config-file with test-only configuration explicitly into
the utility.

neutron.conf.test was renamed into neutron.conf since for some reason
oslo.config does not support a name that does not have .conf at its
filename end.

Change-Id: I719829fc83a7b20a49c338aaf1dbef916dcc768c