nova-network: 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.

This change updates the docstrings for the base API and nova-network API
methods and updates the nova-network API unit tests to use instance
objects when testing allocate_for_instance.

Part of blueprint compute-manager-objects-juno

Change-Id: Ia6d1d8dd46820d48cd9a3f3d4e98fb48e85d1188
Merge "nova-network: treat instance like object in allocate_for_instance"