provider/dummy: Allow IPv6 addresses for instances and storage

Added SetPreferIPv6Addresses() method on the dummy provider
that controls whether to add IPv6 addresses to started
instances and what storage URLs to use (IPv4 or IPv6).
Also sets the network.PreferIPv6 flag, which influences
addresses selection to prefer IPv6 addresses when available.
1) may give invalid address if estate.maxId+1 > 0xffff
2) fc00:: is reserved prefix, I would use fe80:: for that purpose (autoconfigured IPv6 addresses always contain ff:fe inside, so they will be distinguished: fe80::a00:27>>>ff:fe<<<00:3001/64