config validator requirements

1. Integrate with existing validation hook in the service class http://git.openstack.org/cgit/openstack/nova/tree/nova/service.py#n283
2. Minimize performance effect on CI gate tests
3. Consider upgrade implications, integrate with upgrade testing tools
4. Stand-alone tool which will also work across services
5. Discover components to validate configurations for (e.g. if Neutron is configured, then check Neutron credentials are correct)
6. Stand-alone tool which will be run after initial deployment
7. Track validated configurations and only revalidate if needed
8. Run validation during service startup unless that affects performance
9. catch following error types: a) duplicate/nonexistant options; b) invalid mix of settings
10. generic standalone tool for case a) is in nova/tools: https://git.openstack.org/cgit/openstack/nova/tree/tools/config/analyze_opts.py
11. encode validation options in config option declaration
12. library to be reused between standalone tool and service init-hook
13. coordinate with https://etherpad.openstack.org/p/icehouse-oslo-config-import-side-effects
14. integrate with existing sample config file generator
15. integrate with generate documentation from config files and transform constraints into human-readable version

Note that the code to auto-generate documentation from config files is in: http://git.openstack.org/cgit/openstack/openstack-manuals/tree/tools/autogenerate-config-docs