neutron-db-manage finds automatically config file

This patch lets oslo.config find config file containing connection
string to database by itself. Config file can be overriden by
--config-file cli parameter as it was before, so it is backward
compatible.

Change-Id: Iacbcd65edfe9ef268a0caf80641e0937a99b06a7
Closes-Bug: 1381100

neutron/db/migration/cli.py

@@ -172,6 +172,6 @@ def main():
     # attach the Neutron conf to the Alembic conf
     config.neutron_config = CONF
 
-    CONF()
+    CONF(project='neutron')
     #TODO(gongysh) enable logging
     CONF.command.func(config, CONF.command.name)