add new site-id and custom-url options

This will add two new options, `site-id` and `custom-url`. Used in tandem, you can have RF runs over some ad-hoc staging or QA environments.

This is a WIP and just adds the options and doc, it does nothing yet.
Didn't know about that operator, though, like @ukd1 said, `site-id` should be usable alone, so going to adjust for that.

Right now the API will filter tests by `site_id` when receiving it. Do you think we should change that, add a new kind of parameter for the API?

I also think we should be able to set custom env per site. Going to look at a better solution, more future proof, for that.