Add haproxy_backend_only flag to service template

Addition of a `haproxy_backend_only` flag to the service template
for instances in which a frontend service uses ACLs to propgate
requests to multiple backend services, and not all backend services
require a corresponding frontend. This should prevent the
proliferation of spurious frontend services.

Change-Id: I8c419be82cffd289ffcc5086afac923d6eb1a78a

doc/source/configure-haproxy.rst

@@ -356,3 +356,16 @@ Here is an example that shows how to achieve the goal
 This will add two acl rules ``path_sub -i write`` and ``path_sub -i query``  to
 the front end and use the backend specified in the rule. If no backend is specified
 it will use a default ``haproxy_service_name`` backend.
+
+If a frontend service directs to multiple backend services using ACLs, and a
+backend service does not require its own corresponding front-end, the
+`haproxy_backend_only` option can be specified:
+
+.. code-block:: yaml
+
+  - service:
+        haproxy_service_name: influxdb
+        haproxy_backend_only: true # Directed by the 'influxdb-relay' service above
+        haproxy_backend_nodes:
+          - name: influxdb-service
+            ip_addr: 10.100.10.10

templates/service.j2

@@ -29,6 +29,7 @@
   {% set vip_binds = item.service.haproxy_bind %}
 {% endif -%}
 
+{% if not item.service.haproxy_backend_only | default(false) %}
 {% for vip_bind in vip_binds %}
 {% if item.service.haproxy_redirect_http_port is defined and item.service.haproxy_ssl %}
 {% if (loop.index == 1 or item.service.haproxy_ssl_all_vips | default(false) | bool) %}
@@ -78,6 +79,7 @@ frontend {{ item.service.haproxy_service_name }}-front-{{ loop.index }}
     mode {{ item.service.haproxy_balance_type }}
     default_backend {{ item.service.haproxy_service_name }}-back
 {% endfor %}
+{% endif %}
 
 {% set backend_options = item.service.haproxy_backend_options|default([]) %}
 {% set backend_arguments = item.service.haproxy_backend_arguments|default([]) %}