Make servers::update() use Instance.save() to do the work

This makes the update() api call operate on the instance object
directly, by calling update() and save(), instead of calling into
compute_api to do the work.

This requires an extension to a previous kludge for api_samples
to make sure that the database gets updated in sync with the
values we're stubbing out for network information.

This also requires a tweak to one of the tests to return an
updated fake instance object directly from the database, instead
of just from the compute_api.

Related to blueprint compute-api-objects

Change-Id: I62b4009ef97ea47b20442a149ba172de4e343e15
Merge "Make servers::update() use Instance.save() to do the work"