Clarify precedence of secret_key_file
Clarify precedence of secret_key_file

If secret_key_file is set, then secret_key is ignored. Clarify this in
the help strings.

Change-Id: Ie10ebb7c76e274b31ab9826a81373518c9b09e0a

JioCloud/oslo-incubator,JioCloud/oslo-incubator,JioCloud/oslo-incubator