Import openstack.common.config from oslo-incubator

This imports the automatic config file generator code from
oslo-incubator, at commit 3756985c4538c322f98e55650c727c1f066334b6.

Change-Id: Ia655624a62caf543fc4f6ecd5cfe62363781b666
Merge "Import openstack.common.config from oslo-incubator"