'Checks the existence of an instance.
Checks the existence of an instance. This is an override of the
base method for efficiency.
:param instance: The instance object.
:returns: True if the instance exists. False if not.'

def instance_exists(self, instance):

    try:
        self._validate_instance_and_node(instance)
        return True
    except exception.InstanceNotFound:
        return False
