neutronv2: treat instance like object in allocate_for_instance

The allocate_for_instance method gets an instance object from the
compute manager so treat it like an object by using dot notation.

Updates the existing unit tests to use an instance object.

Part of blueprint compute-manager-objects-juno

Change-Id: Ia9305a67d1b9bd8601443da3288f710ab02b3f37
Merge "neutronv2: treat instance like object in allocate_for_instance"