Refactor compute_api reboot tests to be unit-y

This follows previous efforts to unittest-ify the reboot
tests for compute_api. This makes the cells testing much
cleaner, and will massively simplify testing with objects.

Related to blueprint compute-api-objects

Change-Id: I5883f74c613888ed5f112ac1b7b1521f43644768
Merge "Refactor compute_api reboot tests to be unit-y"