Make sure we actually run haveged on our slaves

We bother installing it, which on ubuntu makes sure the service is
running. But on centos, things don't start unless you tell them to.
Tell puppet to tell centos to tell haveged to run so that we can have
entropy.

Change-Id: Ib4ffb00b30123e3b29c7e92466c5060165a83b19

manifests/slave.pp

@@ -194,4 +194,9 @@ class jenkins::slave(
     group  => 'root',
     mode   => '0755',
   }
+
+  service { 'haveged':
+    ensure  => 'running',
+    require => Package[$::jenkins::params::haveged_package],
+  }
 }