blueprint execution: Make inter-step dependency explicit

[jgallagher]

Zone cleanup and saga reassignment both assume that expunged zones in the blueprint are no longer running, which is currently dependent on the earlier deploy_zones execution step completing successfully. Add an empty DeployZonesDone token to make this dependency explicit.

This is a small change with no runtime effect that will prevent us from closing #6999 before we address these steps.

[jgallagher]

Update: after chatting with @smklein, I moved the support bundle reassignment later and made it also dependent on the deploy zones step.

[smklein]

This fixes #7529

[sunshowers]

Neat!

Are we going to need to make this determination on a per-sled basis in the future? I'm wondering if we can spit out a graphviz dot file somehow.

[jgallagher]

Are we going to need to make this determination on a per-sled basis in the future? I'm wondering if we can spit out a graphviz dot file somehow.

I don't think so; I think the preference is to break these dependencies entirely. This will require execution being able to tell the difference between "expunged but might still be running" and "expunged and guaranteed to no longer be running", which itself requires the planner to consult inventory and annotate the blueprint when a thing is guaranteed to no longer be running. #7286 does this for physical disk decommissioning by adding an explicit state in the blueprint disk config. In a call earlier this week, we talked about an alternative: we could BlueprintZoneDisposition::Expunged to something like BlueprintZoneDisposition::Expunged { eligible_for_cleanup: bool }. The planner is responsible for filling that in with true once it confirms via inventory that some prior execution has succeeded sufficiently that we know the sled where this zone was running is no longer running it (and will never try to rerun it).