Validate Flux flags before running cluster create

[Callisto13]

Part of this plan.

One thing we lose by having these arbitrary fields is the extra validation: are users setting the correct flags for their chosen git provider? Of course Flux will complain if you get the flags wrong, and we will be directing users to the Flux docs so that they know what options are available to them, but I can see people tripping on this and it is really annoying to get to the end of a 25 min cluster create, only to have your flux bootstrap fail.

Ideally we could "validate" the flags are at least applicable to the bootstrap command before starting the create.

Flux doesn't have a dry-run bootstrap option (AFAIK), so we cannot call the command without actually running it. But we could hack it by maybe appending a garbage flag to the end of the list we pass in. Flux will fail on the first flag it does not recognise, so if it errors on anything other than our nonsense flag, then the validation is failed.

I am not 100% sold on this, and we don't need it right away, but I think it may be neat to have something like it.