Issue description

health Check plugin in base on Kong lua-resty-healthcheck.

I need a function that can mark an upstream server as backup.
It will be passed requests when only the primary servers are unavailable.

##LIKE NGINX UPSTREAM CONFIG

upstream  testproxy  {
        server   192.168.1.14:8080;
        server   192.168.1.200:8080 backup;
}

USE nginx upstream on http://nginx.org/en/docs/http/ngx_http_upstream_module.html

Environment

• apisix version (cmd: apisix version): 1.4
• OS: centos7