Avoid a race when associating instance_uuid

Current code can potentially overwrite the instance_uuid without
exception when multiple dbapi.update_node calls are issued. This
patch adds a check to dbapi so that the NodeAssociated exception
will be raised in this case.
The Nova Ironic driver will handle this exception. This will be
introduced in a separate patch.

Change-Id: I28ef5ce55399bde7073ffc111e9bc4400e5577ee
Partial-Bug: #1244541
Merge "Avoid a race when associating instance_uuid"