Currently, when users want to get the sample config file, they
need to install many development tools such as git, tox and etc,
then run `tox -egenconfig` to get it.
oslo_config provides a tool named sphinxconfiggen since version
2.3.0, which enables us generate the sample config file along with
document.
Also the OpenStack Manual Configuration Reference doesn't have
sample config file for master branch, the stable branches need to
be improved as well, so this paragraph is removed.
Change-Id: Ie61beef490618f18a711e789cb1ac006b85b8f0a
821f0dd1c5