ami-6b6f4b0e ubuntu/images-testing-dev/hvm-ssd/ubuntu-xenial-16.04-testing-amd64-server-20170307
ami-6a6f4b0f ubuntu/images-testing/hvm-ssd/ubuntu-yakkety-daily-amd64-server-20170307

I launched an instance in an EC2 region (us-east-2) into a subnet with configured ipv6.

$ rel=$(lsb_release -sc)
$ line=$(awk '$1 == "deb" && $2 ~ /ubuntu.com/ { printf("%s %s %s-proposed main universe\n", $1, $2, rel); exit(0) }; ' "rel=$rel" /etc/apt/sources.list)
$ echo "$line" | sudo tee /etc/apt/sources.list.d/proposed.list
$ sudo apt-get update -q && sudo apt-get install cloud-init -qy </dev/null
$ dpkg-query --show cloud-init
cloud-init  0.7.9-48-g1c795b9-0ubuntu1~16.10.1

## just show the network info to show the ipv6 section.
$ python3 -c 'from cloudinit import ec2_utils; print(ec2_utils.get_instance_metadata()["network"])'
{'interfaces': {'macs': {'06:20:c9:69:43:c3': {'public-ipv4s': '52.14.184.32', 'vpc-ipv6-cidr-blocks': '2600:1f16:aeb:b200::/56', 'subnet-id': 'subnet-20b8565b', 'interface-id': 'eni-8388dff8', 'owner-id': '950047163771', 'subnet-ipv4-cidr-block': '172.31.16.0/20', 'subnet-ipv6-cidr-blocks': '2600:1f16:aeb:b20b::/64', 'security-groups': 'wide-open', 'security-group-ids': 'sg-5a61d333', 'local-hostname': 'ip-172-31-30-202.us-east-2.compute.internal', 'ipv4-associations': {'52.14.184.32': '172.31.30.202'}, 'vpc-ipv4-cidr-blocks': '172.31.0.0/16', 'vpc-ipv4-cidr-block': '172.31.0.0/16', 'ipv6s': '2600:1f16:aeb:b20b:7b8c:976e:5010:f218', 'vpc-id': 'vpc-87e72bee', 'mac': '06:20:c9:69:43:c3', 'public-hostname': 'ec2-52-14-184-32.us-east-2.compute.amazonaws.com', 'local-ipv4s': '172.31.30.202', 'device-number': '0'}}}}


I then set default networking for cloud-init to attempt both ipv4 and ipv6.

$ ADDR=$(cat /sys/class/net/eth0/address)
$ sudo tee /etc/cloud/cloud.cfg.d/99-network-config.cfg <<EOF
network:
 version: 1
 config:
  - type: physical
    name: eth0
    mac_address: $ADDR
    subnets:
     - type: dhcp
     - type: dhcp6
EOF

Then, clean the instance, and reboot.

$ sudo rm -Rf /var/log/cloud-init /var/lib/cloud
$ sudo reboot

## When you go back in, look /etc/network/interfaces.d/50-cloud-init.cfg
## and verify it has an ipv6 address and a dhclient6 running.
$ grep -v "^[#]." /etc/network/interfaces.d/50-cloud-init.cfg  | grep .
auto lo
iface lo inet loopback
auto eth0
iface eth0 inet dhcp
iface eth0 inet6 dhcp

$ grep INFO.*network /var/log/cloud-init.log  | head -n 1
2017-03-08 20:44:20,861 - stages.py[INFO]: Applying network configuration from system_cfg bringup=False: {'version': 1, 'config': [{'name': 'eth0', 'type': 'physical', 'mac_address': '06:20:c9:69:43:c3', 'subnets': [{'type': 'dhcp'}, {'type': 'dhcp6'}]}]}


$ ip address show dev eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9001 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 06:20:c9:69:43:c3 brd ff:ff:ff:ff:ff:ff
    inet 172.31.30.202/20 brd 172.31.31.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 2600:1f16:aeb:b20b:7b8c:976e:5010:f218/128 scope global 
       valid_lft forever preferred_lft forever
    inet6 fe80::420:c9ff:fe69:43c3/64 scope link 
       valid_lft forever preferred_lft forever

$ ps axw | grep [d]hclient
  777 ?        Ss     0:00 /sbin/dhclient -1 -v -pf /run/dhclient.eth0.pid -lf /var/lib/dhcp/dhclient.eth0.leases -I -df /var/lib/dhcp/dhclient6.eth0.leases eth0
 1050 ?        Ss     0:00 /sbin/dhclient -1 -6 -pf /run/dhclient6.eth0.pid -lf /var/lib/dhcp/dhclient6.eth0.leases -I -df /var/lib/dhcp/dhclient.eth0.leases eth0