MB-11774: allow safe incoming per-node keys

Particularly, membership is updated during rebalance by master
node. And we anticipate other nodes updating rest and capi_port keys.

Change-Id: I57ade2741f81e3563c6cae0f6e7c3fc4d20d1d6c
Reviewed-on: http://review.couchbase.org/39620
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>