Automatically generate neutron core configuration files

Automatically generate neutron core configuration files

This adds a new tox environment, genconfig, which generates sample
neutron core configuration file using oslo-config-generator.

Updates to some configuration option help messages to reflect useful
details that were missing in the code but were present in config files.

It also adds details to devref on how to update config files.

Partially-Implements: blueprint autogen-neutron-conf-file

DocImpact

Change-Id: I1c6dc4e7d479f1b7c755597caded24a0f018c712
Closes-bug: #1199963
Co-Authored-By: Louis Taylor <louis@kragniz.eu>