'Applies a diff to the instance metadata.
This is an optional driver method which is used to publish
changes to the instance\'s metadata to the hypervisor.  If the
hypervisor has no means of publishing the instance metadata to
the instance, then this method should not be implemented.
:param context: security context
:param instance: nova.objects.instance.Instance'

def change_instance_metadata(self, context, instance, diff):

    pass
