Fix hardcoded backend hosts in http-proxy apache config

Description

In the http-proxy apache config roles/http_proxy/templates/front-end_conf.j2, the ProxyPassReverse section hard-coded the values for backend hosts. Since we are using VMs deployed in the cloud as backends for OOD etc. behind a web-proxy, we need a way to define them in the web-proxy's Apache config.

Proposed solution

Use Jinja templating for loop to parameterize them.