force ~backuppc/.ssh/known_hosts to /dev/null

use the system puppet maintained ssh_known_hosts
diff --git a/manifests/server.pp b/manifests/server.pp
index 4acb8ac..b832c65 100644
--- a/manifests/server.pp
+++ b/manifests/server.pp
@@ -413,6 +413,12 @@
 
     'topdir_ssh':
       path => "${real_topdir}/.ssh",;
+
+    'topdir_ssh_known_hosts':
+      path   => "${real_topdir}/.ssh/known_hosts",
+      ensure => 'link',
+      target => '/dev/null',
+      force  => 'true';
   }
 
   # Workaround for client exported resources that are