Fix oslo.service config generation

Fix oslo.service config generation

Cinder generation configuration sample does not contain any of
oslo.service configuration options.

This is because we are not using the right namespace when calling
oslo-config-generator since oslo.service is not a valid entrypoint as
mentioned in the documentation:

https://github.com/openstack/oslo.service/blob/master/doc/source/usage.rst#using-osloservice-with-oslo-config-generator

This patch fixes this by using the right namespaces.

Closes-Bug: #1552689
Change-Id: I8365daec0d3b161d3c07f229d221cf004edb6662