Add way to change weight of haproxy backend per service

This patch adding option to control weight of haproxy
backends per service via host variable.

Example:

[control]
server1 haproxy_nova_api_weight=10
server2 haproxy_nova_api_weight=2 haproxy_keystone_internal_weight=10
server3 haproxy_keystone_admin_weight=50

If weight is not defined, everything is working as before.

Change-Id: Ie8cc228198651c57f8ffe3eb060875e45d1f0700

ansible/roles/haproxy-config/templates/haproxy_single_service_split.cfg.j2

@@ -93,7 +93,12 @@ backend {{ service_name }}_back
         {% for host in groups[host_group] %}
             {% set host_name = hostvars[host]['ansible_hostname'] %}
             {% set host_ip = 'api' | kolla_address(host) %}
-    server {{ host_name }} {{ host_ip }}:{{ listen_port }} {{ haproxy_health_check_final }} {{ backend_tls_info }}
+    {% set service_weight = 'haproxy_' + service_name + '_weight' %}
+    {% set backend_weight_info = '' %}
+    {% if hostvars[host][service_weight] is defined and hostvars[host][service_weight] | int != 0 and hostvars[host][service_weight] <= 256 %}
+        {% set backend_weight_info = 'weight %s'|format(hostvars[host][service_weight]) %}
+    {% endif %}
+    server {{ host_name }} {{ host_ip }}:{{ listen_port }} {{ haproxy_health_check_final }} {{ backend_tls_info }} {{ backend_weight_info }}
         {% endfor %}
     {% endif %}
 {% endmacro %}

doc/source/reference/high-availability/haproxy-guide.rst

@@ -45,3 +45,20 @@ This is especially helpful for connections to MariaDB. See
 `here <https://blog.cloudflare.com/when-tcp-sockets-refuse-to-die/>`__ and
 `here <https://access.redhat.com/solutions/726753>`__ for
 further information about this kernel option.
+
+Backend weights
+---------------
+
+When different baremetal are used in infrastructure as haproxy backends
+or they are overloaded for some reason, kolla-ansible is able to change
+weight of backend per sevice. Weight can be any integer value from 1 to
+256.
+
+To set weight of backend per service, modify inventory file as below:
+
+.. code-block:: ini
+
+   [control]
+   server1 haproxy_nova_api_weight=10
+   server2 haproxy_nova_api_weight=2 haproxy_keystone_internal_weight=10
+   server3 haproxy_keystone_admin_weight=50

releasenotes/notes/haproxy-service-backend-weight-e4a908f732a37b42.yaml

@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    The haproxy-config role now allows user to set weight per
+    haproxy's backend. This can be achieved by setting a hostvar
+    ``haproxy_{{ service }}_weight`` in inventory file to any integer
+    value in range from 1 to 256, so the higher the weight, the higher
+    the load. This can be set per ``{{ service }}``. If hostvar is not
+    specified, backend's weight is not rendered in final haproxy
+    configuration.