dhcpv6-pd: T421: workaround for non existing interfaces

When DHCPv6-PD is configured to delegate a prefix to a non existing interface,
it is restarted (systemd default) but will then hit the restart rate-limit which
disables the service entirely.

As VyOS currently has no "hook" to be called once an interface goes online we
need this "try and error" approach until there is a way to deal with it. This
behavior can be reproduced when delegating an IPv6 prefix to a bridge interface
as a bridge interface will always be started after all interfaces have been
configured.

We will now restart dhcp6c as long as the requested interface is online.