Allow to detect which node gave current configuration

This feature used in libcouchbase to judge about healthness of REST
connection.

Change-Id: Ie6370ca2d6e7313228965870d70d419e6b79a0f5
Reviewed-on: http://review.couchbase.org/30833
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>