Modify settings for a DB instance.
    CLI example to description of parameters ::

        salt myminion boto_rds.modify_db_instance db_instance_identifier region=us-east-1

def modify_db_instance(name,
                       allocated_storage=None,
                       allow_major_version_upgrade=None,
                       apply_immediately=None,
                       auto_minor_version_upgrade=None,
                       backup_retention_period=None,
                       ca_certificate_identifier=None,
                       character_set_name=None,
                       copy_tags_to_snapshot=None,
                       db_cluster_identifier=None,
                       db_instance_class=None,
                       db_name=None,
                       db_parameter_group_name=None,
                       db_port_number=None,
                       db_security_groups=None,
                       db_subnet_group_name=None,
                       domain=None,
                       domain_iam_role_name=None,
                       engine_version=None,
                       iops=None,
                       kms_key_id=None,
                       license_model=None,
                       master_user_password=None,
                       monitoring_interval=None,
                       monitoring_role_arn=None,
                       multi_az=None,
                       new_db_instance_identifier=None,
                       option_group_name=None,
                       preferred_backup_window=None,
                       preferred_maintenance_window=None,
                       promotion_tier=None,
                       publicly_accessible=None,
                       storage_encrypted=None,
                       storage_type=None,
                       tde_credential_arn=None,
                       tde_credential_password=None,
                       vpc_security_group_ids=None,
                       region=None, key=None, keyid=None, profile=None):
    '''
    Modify settings for a DB instance.
    CLI example to description of parameters ::

        salt myminion boto_rds.modify_db_instance db_instance_identifier region=us-east-1
    '''
    res = __salt__['boto_rds.exists'](name, tags=None, region=region, key=key, keyid=keyid, profile=profile)
    if not res.get('exists'):
        return {'modified': False, 'message':
                'RDS db instance {0} does not exist.'.format(name)}

    try:
        conn = _get_conn(region=region, key=key, keyid=keyid, profile=profile)
        if not conn:
            return {'modified': False}

        kwargs = {}
        excluded = set(('name',))
        boto_params = set(boto3_param_map.keys())
        keys = set(locals().keys())
        for key in keys.intersection(boto_params).difference(excluded):
            val = locals()[key]
            if val is not None:
                mapped = boto3_param_map[key]
                kwargs[mapped[0]] = mapped[1](val)

        info = conn.modify_db_instance(DBInstanceIdentifier=name, **kwargs)

        if not info:
            return {'modified': bool(info), 'message':
                    'Failed to modify RDS db instance {0}.'.format(name)}

        return {'modified': bool(info), 'message':
                'Modified RDS db instance {0}.'.format(name),
                'results': dict(info)}
    except ClientError as e:
        return {'error': __utils__['boto3.get_error'](e)}