MB-8663: implemented just_validate for replication settings

Change-Id: I86d07e128160f49cdfc12b207162d78f62472a4f
Reviewed-on: http://review.couchbase.org/28353
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>