marsadm: show the old designated primary in the log

This is vital for incident analysis.

userspace/marsadm

@@ -2104,6 +2104,7 @@ sub primary_phase0 {
     check_todo($cmd, $res, "fetch", 0, 0);
   }
   my $old = _get_designated_primary($res);
+  lprint "Current designated primary: $old\n";
   if ($cmd eq "primary") {
     check_sync_finished($res, $host) if $host ne $old;
     # also check that other secondaries won't loose their sync primary