Gather facts from the hosts before using them

Haproxy needs to have gathered facts from all hosts that it will use
information about. In this case it must talk to all of the api hosts
as well as the database and rabbitmq hosts before generating the
configuration file.

Change-Id: I99b7dbebd5a6193e192ee258ddf576d18db90ed7
Closes-Bug: #1482406

ansible/roles/haproxy/tasks/main.yml

 ---
 - include: config.yml
+  when: inventory_hostname in groups['haproxy']
 
 - include: start.yml
+  when: inventory_hostname in groups['haproxy']

ansible/site.yml

 ---
-- hosts: haproxy
+- hosts: [haproxy, mariadb, rabbitmq, cinder-api, glance-api, keystone, nova-api, neutron-server, swift-proxy-server]
   roles:
     - { role: haproxy, tags: haproxy, keepalived, when: enable_haproxy | bool }