design: support post-upgrade hooks

While the general policy for Ganeti is to just accept the situation
it finds after being upgraded from an older version, in some cases
additional actions might be necessary. So support a hook for doing
so.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Petr Pudlak <pudlak@google.com>

doc/design-upgrade.rst

@@ -227,6 +227,8 @@ following actions.
 
 - The intent-to-upgrade file is removed.
 
+- ``post-upgrade`` is run with the original version as argument.
+
 - ``gnt-cluster verify`` is run and the result reported.