Refactor test_rescue_unrescue into compute api/manager unit tests

As part of converting the rescue/unrescue compute API flows to using
objects, i.e. dot notation for accessing attributes on the instance
object and updating the database via instance.save() rather than
compute_api.update, we want to move some of the integration-style tests
found in test_compute.py to test_compute_api.py and test_compute_mgr.py.

This is to help with the conversion so we don't have to use exotic stubs
in the cells API flows to make integration tests work.

This change is mainly concerned with moving test_rescue_unrescue out of
test_compute.py and into the api/manager specific test modules and then
later changes will build on top of the new tests.

Part of blueprint compute-manager-objects-juno

Change-Id: I2a9b1c6b5a1f41272e26c20de134a2a8b71c88f3
Merge "Refactor test_rescue_unrescue into compute api/manager unit tests"