We detect an IPv6 vs V4 address based on columns, rather than passing
the family from the cluster object to be more future proof (in case
we'll ever support mixed clusters).

Unfortunately quite a bit more code is required to test this: we need an
arbitrary node that does the right thing w.r.t. ip addresses and also
test-only exports. As such we'll do this out of the stable branch.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michele Tartara <mtartara@google.com>