Wrong handling of Instance expected_task_state

When Instance.save called the expected_task_state parameter can be
passed to it in order to specify what is the expected task state.
There many cases when we would expect that there no task state
for the vm (task_state=None in db). Currently the
expected_task_state=None is overwritten by the default substitution in
the save() arguments and due to this not passed correctly to the
db.instance_update_and_get_original method call.

Closes-Bug: #1225659

Change-Id: I84fd2f7e2c50c5f34c84f713d6afc040524a8078
Merge "Wrong handling of Instance expected_task_state"