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.
MgoServer.Addr() always returns "localhost:<random port>", so this is keeping that endpoint as a second choice and adding the preferred IPv6 one as first choice, to better emulate dual stack setups.