Better way to ensure keepalived before haproxy.

The lastest patchset of https://review.openstack.org/393361 was actually
not working.

The `if defined` idiom depends on *evaluation* order.

At the time it's red in the haproxy.pp class, the line that loads the
class 'haproxy' has still not yet been reached and thus the `defined`
result is false. The constraint is not added.

For this reason, the use of `defined` in module is not advised by
puppetlabs[1].

[1] https://docs.puppet.com/puppet/latest/reference/function.html#defined

Change-Id: Ibd352cb313f8863d62db8987419378bed5b87256
Relates-To: #1638029

manifests/haproxy.pp

@@ -599,6 +599,8 @@ class tripleo::haproxy (
   # This code will be removed once we switch undercloud and overcloud to use both haproxy & keepalived roles.
   if $keepalived {
     include ::tripleo::keepalived
+    # Make sure keepalive starts before haproxy.
+    Class['::keepalived::service'] -> Class['::haproxy']
   }
 
   # TODO(bnemec): When we have support for SSL on private and admin endpoints,

manifests/keepalived.pp

@@ -158,8 +158,4 @@ class tripleo::keepalived (
       priority     => 101,
     }
   }
-  # Make sure keepalive starts before haproxy.
-  if (defined(Class['::haproxy'])) {
-    Class['::keepalived::service'] -> Class['::haproxy']
-  }
 }