IPv6-enabled EC2 subnets always have dhcp enabled

[Major Hayden]

A Fedora user opened an issue[0] about an issue with cloud-init on EC2. When they launch an instance on a IPv6-enabled subnet, cloud-init sets the `ipv6.method` to `dhcp` in NetworkManager.

However, AWS uses router advertisements to let instances know about the nearest router. Using the `ipv6.method: dhcp` setting prevents router advertisements from working. The instance ends up with an IPv6 address assigned, but it cannot route traffic.

A workaround is to run NetworkManager commands to fix the issue:

```
nmcli con edit CONNECTION_UUID
nmcli> set ipv6.method auto
nmcli> save
nmcli> activate
```

The instance immediately routes IPv6 traffic after making that change. The dhcp setting appears to be applied automatically for EC2 subnets with IPv6 addresses assigned[1].

I would expect `ipv6.method` to be `auto` in these situations.

Thank you!

[0] https://pagure.io/cloud-sig/issue/382
[1] https://github.com/canonical/cloud-init/blob/53a995e2f852d043d51ad25c1b9afbbe1edafd57/cloudinit/sources/DataSourceEc2.py#L861-L863

[mhayden (mhayden-redhat-bugs)]

A Fedora user opened an issue[0] about an issue with cloud-init on EC2. When they launch an instance on a IPv6-enabled subnet, cloud-init sets the `ipv6.method` to `dhcp` in NetworkManager.

However, AWS uses router advertisements to let instances know about the nearest router. Using the `ipv6.method: dhcp` setting prevents router advertisements from working. The instance ends up with an IPv6 address assigned, but it cannot route traffic.

A workaround is to run NetworkManager commands to fix the issue:

```
nmcli con edit CONNECTION_UUID
nmcli> set ipv6.method auto
nmcli> save
nmcli> activate
```

The instance immediately routes IPv6 traffic after making that change. The dhcp setting appears to be applied automatically for EC2 subnets with IPv6 addresses assigned[1].

I would expect `ipv6.method` to be `auto` in these situations.

Thank you!

[0] https://pagure.io/cloud-sig/issue/382
[1] https://github.com/canonical/cloud-init/blob/53a995e2f852d043d51ad25c1b9afbbe1edafd57/cloudinit/sources/DataSourceEc2.py#L861-L863

[James Falcon (falcojr)]

Tracked in Github Issues as https://github.com/canonical/cloud-init/issues/3980