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) That's true, but we never do that in unit tests.
2) fe80:: is "link-local" and specifically excluded when selecting an internal or public address, fc00::/7 is the range reserved for unique local addresses (IPv6 private addresses), which are considered "local-cloud".
The dummy provider is used for testing, so we might be a bit less strict with enforcing sanity checks on generated ids and addresses. It's better to have something than nothing for now :)