Fix haproxy prechecks after deployment

HAProxy prechecks could fail if the ansible_user was not allowed
to access Docker API.

Change-Id: I09bfa35392bed77321d2de2424e44e60b60a8451

ansible/roles/loadbalancer/tasks/precheck.yml

@@ -194,10 +194,10 @@
     - inventory_hostname in groups['loadbalancer']
 
 - name: Getting haproxy stat
+  become: true
   shell: echo "show stat" | docker exec -i haproxy socat unix-connect:/var/lib/kolla/haproxy/haproxy.sock stdio  # noqa 306
   register: haproxy_stat_shell
   changed_when: false
-  failed_when: false
   when: container_facts['haproxy'] is defined
 
 - name: Setting haproxy stat fact