Correctly handle change of key in ns_config:update.

Since key can change, we need to remove potential duplicates.
Otherwise we may see old value.

Change-Id: I5399dd942ac7b70c497b73e42830f93327252448
Reviewed-on: http://review.couchbase.org/39500
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>