This commit adds sample config file generation to the nova devref
using the oslo.config sphinxconfiggen module which was recently added.
This will generate a new sample config each time build sphinx is run.
This is then used on a new docs page where you can either view the file
in its entirety, or download the file. The sphinx module was added in
the oslo.config 2.3.0 release.
Change-Id: I6d9150d81c8204bee8f775021a854928671bdd02
9a3ed7631a