plugable: support plugin versioning

Allow multiple incompatible versions of a plugin using the same name. The
current plugins are assumed to be version '1'.

The unique identifier of plugins was changed from plugin name to plugin
name and version. By default, the highest version available at build time
is used. If the plugin is an unknown remote plugin, version of '1' is used
by default.

https://fedorahosted.org/freeipa/ticket/4427

Reviewed-By: David Kupka <dkupka@redhat.com>

ipaserver/rpcserver.py

@@ -673,7 +673,8 @@ class xmlserver(KerberosWSGIExecutioner):
         """list methods for XML-RPC introspection"""
         if params:
             raise errors.ZeroArgumentError(name='system.listMethods')
-        return (tuple(unicode(cmd.name) for cmd in self.Command()) +
+        return (tuple(unicode(cmd.name) for cmd in self.Command()
+                      if cmd is self.Command[cmd.name]) +
                 tuple(unicode(name) for name in self._system_commands))
 
     def _get_method_name(self, name, *params):