Prevent possible http_mod headers declaration conflict

Making a minor tweak that allows graphite to be deployed on a node
that also has other resources that need to use the headers http_mod,
for instance jenkins.  See:
http://git.openstack.org/cgit/openstack-infra/puppet-jenkins/tree/manifests/master.pp#n66
for an example where the same approach is used.

Change-Id: Ia7495f7640d356f130381fbb021f1cae05e72c00

manifests/init.pp

@@ -209,8 +209,10 @@ class graphite(
     template => 'graphite/graphite.vhost.erb',
   }
 
-  httpd_mod { 'headers':
-    ensure => present
+  if !defined(Httpd::Mod['headers']) {
+    ::httpd::mod { 'headers':
+      ensure => present,
+    }
   }
 
   vcsrepo { '/opt/statsd':