change postfix senders file to use FQDN rather than hostname

NEWS

 unreleased (207x-xx-xx)
 
+    [postfix] Change the sender address for root mail from
+    "root@${::hostname}" to "root@${::fqdn}". This is needed by newer
+    versions of postfix and makes more sense anyway.
+
     [wallet] Make wallet ketyab use the correct path for ktutil.
 
     [duo] Change apt pin file to point to codename rather than archive

manifests/postfix.pp

@@ -61,7 +61,7 @@ class base::postfix {
 
   # Have mail from root@stanford.edu come from root at the local system
   base::postfix::sender { 'root@stanford.edu':
-    ensure => "root@${::hostname}";
+    ensure => "root@${::fqdn}";
   }
 
   # We used to disable and stop sendmail here on Red Hat, but this doesn't