Set the variant id for this variant

def _set_variant_id(self, variant_id=None):
        """Set the variant id for this variant"""
        if not variant_id:
            variant_id = '_'.join([
                self.CHROM,
                str(self.POS),
                self.REF,
                self.ALT
                ])

        logger.debug("Updating variant id to {0}".format(
            variant_id))

        self['variant_id'] = variant_id