Don't return ets tables' contents to old nodes.

Because old nodes pretty-print the entire value returned by
do_diag_per_node. We saw that it already could cause massive
problems. So it's better not to make it worse.

Change-Id: If213d494c8ef87c17f635f0d78c12f258fbeadc1
Reviewed-on: http://review.couchbase.org/33085
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Artem Stemkovski <artem@couchbase.com>