Having all services in one giant haproxy file makes altering
configuration for a service both painful and dangerous. Each service
should be configured with a simple set of variables and rendered with a
single unified template.

Available are two new templates:

* haproxy_single_service_listen.cfg.j2: close to the original style, but
only one service per file
* haproxy_single_service_split.cfg.j2: using the newer haproxy syntax
for separated frontend and backend

For now the default will be the single listen block, for ease of
transition.

Change-Id: I6e237438fbc0aa3c89a3c8bd706a53b74e71904b