IPv6-only environments need some more love

[yoctozepto]

1. there is no DHCPv6 support as far as I could tell
   a) stateless DHCPv6 (with SLAAC) lacks ability to obtain DNS servers (which would come via DHCPv6 client)
   b) stateful DHCPv6 does not apply at all
2. if no (IPv4) DHCP is present, then there is a long timeout (and irrelevant failures cascade) even though IPv6 SLAAC addressing was configured from enabled-by-default accepting of Router Advertisements (RAs) - this delays the start of ssh daemon (dropbear) which would normally work as it binds to both wildcards anyway; I attach console output [1]

[1] console.txt

[yoctozepto]

Tested in OpenStack CI: https://review.opendev.org/712768

[rmohr]

https://bazaar.launchpad.net/~j-harbott/cirros/dev/changes/383?start_revid=383 from an older discussion in https://bugs.launchpad.net/cirros/+bug/1487041 may be helpful for someone looking into this.

[hrw]

Please check 0.6.0

[yoctozepto]

Checking it:
https://review.opendev.org/c/openstack/kolla-ansible/+/712768
https://review.opendev.org/c/openstack/kolla-ansible/+/859935

[osfrickler]

Passed