add an option to disable dhcpv6 on fallback

# Enhancement
Since PR https://github.com/canonical/cloud-init/pull/4474, dhcpv6 is always enabled in fallback mode. Would it be possible to have a global option to disable it?

I have problems with the fallback mode in eni mode, using ifupdown.
The following configuration is generated:

```
auto ens3
iface ens3 inet dhcp
# control-alias ens3
iface ens3 inet6 dhcp

```

If there is no ipv6 server listening on the interface, the networking service fails with "[FAILED] Failed to start Raise network interfaces.".