WIP: Refactor certificate agent and queue

Details to follow...
Is the way settings does overrides smart enough that you can set these things with environment variables? Because people who don't adopt the ansible infrastructure will find the JSON overrides cumbersome, but like every cloud provider has a way to inject environment variables into your host.

So if settings doesn't, can we fix it easily? And if it does, is it worth adding a bit of additional help on the command line that you can set these things via environment variables?