Add a instance lookup helper to v3 plugins.

There was a lot of duplicated error checking code in the v3 plugins
around instance lookup via the compute API. I've created a generic
helper to do this, and then moved all the existing plugins across to
using that helper. This reduces the amount of duplicated code in
these plugins.

Change-Id: I52c15da0499eda676225603d5a0b2739875d1fca
Merge "Add a instance lookup helper to v3 plugins."