Load balancer appcookie configuration deprecated in HAproxy

##### ISSUE TYPE
* Improvement Request

##### COMPONENT NAME
* ACS 4.11.x VR load balancing with AppCookie stickiness

##### CLOUDSTACK VERSION
* ACS 4.11.x

##### CONFIGURATION
* Advanced zone, guest isolated network with LB configuration on public IP

##### OS / ENVIRONMENT
* N/A

##### SUMMARY
* AppCookie configuration is deprecated in HAproxy
* https://cbonte.github.io/haproxy-dconv/1.7/configuration.html#appsession

##### STEPS TO REPRODUCE
* Create new LB rule with AppCookie stickiness
* Observe haproxy logs on VR stating:
03:59:09 r-1667-VM haproxy[40552]: [ALERT] 090/035909 (40552) : parsing [/etc/haproxy/haproxy.cfg:38] : 'appsession' is not supported anymore, please check the documentation.

##### EXPECTED RESULTS
* N/A

##### ACTUAL RESULTS
* Error as above

从生成的 /etc/haproxy/haproxy.cfg 和 issue 中描述的 appsession 配置开始，然后查看链接的 pull request 以了解当前的处理方式。重现 AppCookie 的会话保持配置，并验证 HAProxy 不再报告不支持 appsession。

由索引模型根据 Issue 内容生成。