Add CLI option support to config fixture

oslo.config.fixture.Config has register_opt() and register_opts() but
not register_cli_opt() and register_cli_opts().

Change-Id: I8a1582dc0deb0230eb02b0894a49398f333375fe
Add oslo-config-generator

Add a sample config file generator utility. This is a dramatically
different approach from the generator in oslo-incubator, whereby we move
away from magic towards much more explicit control over the generator.

blueprint: oslo-config-generator
Closes-Bug: #1300546
Change-Id: I15686708fc9460948a58cfea3d18dae40ba1fda9
Merge "Add CLI option support to config fixture"
Merge "Add oslo-config-generator"