Treat instance like an object in _build_instance

The _build_instance method has an instance object already
but treats it like a primitive dict.  This change uses
dot notation to access fields on the object and updates
the unit tests to use instance objects.

Part of blueprint compute-manager-objects-juno

Change-Id: I19c5abc502b57579b4706e8a8e7b7a64f6f9b6ca
Merge "Treat instance like an object in _build_instance"