oslo.config.generator has been removed from olso-incubator so
we need to adopt it in order to generated up-to-date config
files.
Changes introduced:
- Move config/generator.py from openstack.common to ironic.common
- Update config generator to use new style list_opts discovery
- Split _print_opt function to reduce complexity and pass pep8 tests
Change-Id: I397504bf0604a86082f26e46952720117611f3a8