Clean up references to haproxy_hosts in plays

Instead, reference the 'haproxy' service group. This allows the plays
to be container agnostic and move the haproxy deployment closer to
being container-compatible.

Change-Id: Ibfe0e3a0cb742c8d5c3943922da0a30a56016266

README.rst

@@ -8,7 +8,7 @@ Role for the installation and setup of haproxy
 .. code-block:: yaml
 
     - name: Install haproxy
-      hosts: haproxy_hosts
+      hosts: haproxy
       user: root
       roles:
         - { role: "haproxy_server", tags: [ "haproxy-server" ] }