SystemDescription and ExampleSystem should store Policy structs directly

[sunshowers]

Followup from #6785.

Currently, SystemDescription and ExampleSystem store all the fields from Policy separately. This means that each time we add a policy field, we have to go around adding it to both of these components. It would be really nice to store Policy directly since we expect that knobs become automatically available.

One current challenge with that is that the default Nexus allocation policy in ExampleSystem is one Nexus instance per sled. The Policy struct isn't quite amenable to representing that since it accepts the number of Nexus zones as a usize.

• In response to that, we could change the Policy struct to make the Nexus zone policy more complex than that. But I'm hesitant to do so because production always wants a fixed number of Nexus users.
• Instead, it seems to be easier to update the tests that expect a non-standard number of Nexus zones to set the policy accordingly.