Unregister an AMI.

        :type image_id: string
        :param image_id: the ID of the Image to unregister

        :type delete_snapshot: bool
        :param delete_snapshot: Set to True if we should delete the
                                snapshot associated with an EBS volume
                                mounted at /dev/sda1

        :rtype: bool
        :return: True if successful

def deregister_image(self, image_id, delete_snapshot=False):
        """
        Unregister an AMI.

        :type image_id: string
        :param image_id: the ID of the Image to unregister

        :type delete_snapshot: bool
        :param delete_snapshot: Set to True if we should delete the
                                snapshot associated with an EBS volume
                                mounted at /dev/sda1

        :rtype: bool
        :return: True if successful
        """
        snapshot_id = None
        if delete_snapshot:
            image = self.get_image(image_id)
            for key in image.block_device_mapping:
                if key == "/dev/sda1":
                    snapshot_id = image.block_device_mapping[key].snapshot_id
                    break

        result = self.get_status('DeregisterImage',
                                 {'ImageId':image_id}, verb='POST')
        if result and snapshot_id:
            return result and self.delete_snapshot(snapshot_id)
        return result