Refactor test cases related to instance object

After the blueprint unified-object-model, all of the instances
should be represented by an instance object but not a db
instance dict.

In some test cases, they are still using db instance dict,
we should change those test cases to use instance object.

The fix include three parts:
1) Add a new stub method fake_fault_obj in
nova/tests/fake_instance.py to help convert a dict fault to an
object fault.
2) Add a new stub method fake_instance_obj in
nova/tests/fake_instance.py to help convert a dict instance to
an object instance.
3) Make sure all test cases related to "fault" and "instance" passed.

Closes-Bug 1230125
Partial-Bug 1212915

Change-Id: Ie54fface139e4ad042b9a748c7409c3698875413
Merge "Refactor test cases related to instance object"