Treat instance objects as objects in unrescue API flow

The compute API and manager are getting instance objects now but
treating them like primitive dicts still in several places, so this
cleans that up.

Cleans up the related tests so they are using objects as well to verify
the changes to compute api/manager are correct.

Part of blueprint compute-manager-objects-juno

Change-Id: I81fa858a821a2137fce5970905b9b1c71c95304b
Merge "Treat instance objects as objects in unrescue API flow"