IPaddr2: ipt_CLUSTERIP "iptables" extension not "nft" backend compatible

Reference:
https://lists.clusterlabs.org/pipermail/users/2019-December/026674.html
(thread also sketches a future ambition for a [presumably, to revert
the habit of a functional overloading] separate agent to use
"xt_cluster" extension/cluster match).

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>