Simplify help string for --config-file
Simplify help string for --config-file

As Doug pointed out, we can simplify including the defaults for
--config-file in its help string by using argparse's builtin %(default)s
substitution.

Change-Id: I818101a222a64c12617c89d1f498a0a7c50b2184

citrix-openstack/build-oslo.config,JioCloud/oslo.config,citrix-openstack-build/oslo.config,shad7/oslo.config,metacloud/oslo.config,tianshan/oslo.config,magic0704/oslo.config,varunarya10/oslo.config