propolis-server should use reservoir on prod gear

[pfmooney]

Add "omicron-build" feature to propolis-server which compiles out the developer-friendly vmm-reservoir check, opting to use the reservoir unconditionally on the assumption that Oxide machines will be appropriately configured for such duty.

Fix #408

[gjcolombo]

LGTM. In the future, it'd be great to use the "prod" configuration in CI if we can. I filed #412 for that, but I don't think we need to block on it right now.

[jordanhendricks]

I think we need to wait to merge this until oxidecomputer/omicron#2650 until is done, or at least wait to update the propolis dep in omicron after this is merged. I think the latter will be easier to miss than just not merging this yet.

[pfmooney]

I think we need to wait to merge this until oxidecomputer/omicron#2650 until is done, or at least wait to update the propolis dep in omicron after this is merged. I think the latter will be easier to miss than just not merging this yet.

Yep. I was just rebasing since it had fallen behind.

[gjcolombo]

When we make the change to pick this up in Omicron, we'll also need to change omicron/smf/sled-agent/non-gimlet/config.toml (and gimlet-standalone/config.toml) to specify a nonzero reservoir size, or Omicron dev clusters won't be able to start instances (unless they pick up a Propolis image that isn't built with this feature).