These rules below will allow you to add Ipv6 to all of your instances.. by default openstack only allows ipv6 between the same security group.
openstack security group list
+--------------------------------------+---------+------------------------+----------------------------------+------+
| ID | Name | Description | Project | Tags |
+--------------------------------------+---------+------------------------+----------------------------------+------+
| 9817adcc-e504-479b-97e5-cc884c17d3dc | default | Default security group | 3d1f410104144004a65e74c7d8fa2612 | [] |
| a45b2351-331c-4a71-ab42-10f3f04364f6 | default | Default security group | 877dce1df8ea4f8ba6a28803ef40f0dd | [] |
| afe5f296-1971-49a9-9ef4-f6af98bd83f9 | default | Default security group | 26ef6bf16c1544699b6de2639f006950 | [] |
| def5f039-c5bc-4f0e-a2e0-3ffe537f9bee | default | Default security group | | [] |
| e38fe90f-70b1-4411-984b-ace5b6f04530 | default | Default security group | 776cf3cded7541419baeef3002ebf742 | [] |
+--------------------------------------+---------+------------------------+----------------------------------+------+
#Add the rules to the security group for project "776cf3cded7541419baeef3002ebf742"
openstack security group rule create --protocol ipv6-icmp --ingress e38fe90f-70b1-4411-984b-ace5b6f04530
openstack security group rule create --ethertype ipv6 --protocol tcp --ingress e38fe90f-70b1-4411-984b-ace5b6f04530
openstack security group rule create --ethertype ipv6 --protocol udp --ingress e38fe90f-70b1-4411-984b-ace5b6f04530