add skip-stopped option to fly deploy

[wjordan]

This is a follow-up to #2992, allowing the 'skip stopped machines on deploy' behavior implemented there to be configurable through a --skip-stopped flag in fly deploy, and/or a deploy.skip_stopped property in fly.toml config, so that it can be selectively enabled in additional situations where autoscaling services are not being used.

I'm kind of hesitant to add to the already-huge pile of feature flags that's currently in fly deploy, so I don't know if I'm completely comfortable adding this. However, there is a customer use-case where this kind of configuration does seem to make sense. I'd like some feedback on whether this seems like a good idea.

The configuration is also not per-process-group, which may also limit its usefulness in more complex apps. (Perhaps this points to future work in making the [deploy] section configurable per-process group?)