T5602: Reverse-proxy add option backup for backend server

A backup server can be defined to take over in the case of all other
backends failing

set load-balancing reverse-proxy backend <tag> server <tag> address '192.0.2.3'
set load-balancing reverse-proxy backend <tag> server <tag> port '8883'
set load-balancing reverse-proxy backend <tag> server <tag> backup

(cherry picked from commit cb297aea56da91144c53be1f396b64a26a8e5b04)