Make password hash iterations configurable

[jubrad]

Move password hash iterations to a system var.

Defaults to 600_000 and uses 600_000 for
all system users.

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[jubrad]

While this should be true, I'm open to other ways to do this. It seems like the attribute field needs to be Optional<NonZeroU32>, at least nothing else really makes sense, and this part of the code doesn't have access to dyncfg so it can't fetch a well known default. I suppose we could set to 600_000 with a soft panic?