Add setting to enable ipv6 in docker networks

[tilosp]

For this to work the docker daemon has to be configured for ipv6: https://docs.docker.com/config/daemon/ipv6/

[spantaleev]

I've reworked this to be based on devture_systemd_docker_base_ipv6_enabled, which is not part of the systemd-docker-base role - see here.

I've also added support for auto-reconfiguring our Docker role (geerlingguy/ansible-role-docker) based on the value of devture_systemd_docker_base_ipv6_enabled.

Still, existing installations will probably need to follow the instructions for deleting existing container networks to have changes to devture_systemd_docker_base_ipv6_enabled take effect. New installations with devture_systemd_docker_base_ipv6_enabled: true should work seamlessly - Docker should be auto-reconfigured and all container networks should get IPv6 support.

It should also be noted that not all roles used by this playbook respect the devture_systemd_docker_base_ipv6_enabled variable yet. Many roles are external (installed via ansible-galaxy into roles/galaxy) - none of those respect the devture_systemd_docker_base_ipv6_enabled variable yet, but can be adjusted to do so. They all implicitly depend on the systemd-docker-base role, so they can directly start referencing the new devture_systemd_docker_base_ipv6_enabled variable.