document ganeti retirement procedure and fix retire-all call

tsa/howto/retire-a-host.mdwn

@@ -175,9 +175,12 @@ When you return:
  4. after a delay, retire the host from its parent, backups and
     Puppet, for example:
 
-        ./retire -v -H kvmX.torproject.org retire-all test.torproject.org
+        ./retire -v -H $INSTANCE retire-all --parent-host=$PARENT_HOST
 
-    TODO: cover physical machines and ganeti
+    TODO: `$PARENT_HOST` should be some ganeti node
+    (e.g. `fsn-node-01.torproject.org`) but could be auto-detected...
+
+    TODO: cover physical machines
  5. remove from LDAP with `ldapvi` (STEP 6 above) TODO: add to Fabric,
     make sure you show the diff
  6. do one huge power-grep over all our source code, for example with