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
Merge "neutron-db-manage finds automatically config file"