Add a read_only configuration switch

Given we increasingly use conftool as a library in other software, those might have the need to execute some of their conftool code as a noop, for example during dry runs.

So:

• Add a read_only value to configuration.Config, default to false
• Add the ability to override the configuration on disk in ConftoolClient
• When in read-only mode, do not write to the kvstore, but rather log the action we would've taken