Currently when updating a node we try to acquire the lock on it using
its old database record, including its old driver. This is not correct
when updating a driver, since the current conductor may not have access
to the old driver (and the old driver may no longer be enabled at all).
Since we do not need task.driver at all when updating nodes, this change
stops populating it in update_node.
Change-Id: I510c3bfbd11b01fef05341be2bb04c6bd01bf8ac
Story: #2004741
Task: #28812
cea8d74914