add change remote cluster rest api

Change-Id: Ia6d00adfbc10aa08fa3636331a176df393084b37

handle just_validate param in rest requests

Change-Id: Ia20ef9554139c0a9f987b72ffb81efa5eec04d08
Reviewed-on: http://review.couchbase.org/44399
Reviewed-by: Xiaomei Zhang <xiaomeizhangatwork@gmail.com>
Tested-by: Yu Sui <ysui68@gmail.com>