3 options, all of which require you to have a different dsidentify config file
as you are asking for behavior different than xenial will ever have.

The behavior on xenial is 

a.) configure cloud-init to have strict_id for Ec2.
    This changes the return of DS_MAYBE in ec2 to be DS_NOT_FOUND.

    cat > /etc/cloud/cloud.cfg.d/Ec2-nostrict.cfg <<EOF
    datasource:
      Ec2:
        strict_id: true
    EOF

    The filename can also be /etc/cloud/cloud.cfg.

b.) change your ds-identify config to not allow maybe.
    /etc/cloud/ds-identify.cfg should then have:
        policy=search,found=first,maybe=none,notfound=disable

c.) we can explicitly allow configuration of ds-identify (ds-identify.cfg)
    that would affect its default setting for strict_id.  As it is, by
    design i only allowed for setting that in cloud-init config files
    as duplicating settings like that doesn't seem like a good idea.