oslo.config provides sphinx extensions to generate config reference
and sample config files. This commit enables the auto-generation of
the networking config reference.
Note that this commit just adds the configuration references
for existing genconfig entries. I haven't checked there are options
uncovered. If any, this will be a potential future work.
Part of doc-migration work
Change-Id: I4e150b6045e5e9c56cbe7262b31ee0d82dcb6c69
c314db081e