Make compute_api.get() use objects natively

This finishes what a previous patch started, which makes get()
query instance objects directly.

Most of the change in this patch is just accounting for minor
differences in the unit tests.

Related to blueprint compute-api-objects

Change-Id: Ib8346bcd5a57279a8f6d57adc621910f67ef1b53
Merge "Make compute_api.get() use objects natively"