Fix spurious "Malformed request" error

Since we are invoking _ext_nodes now for backward compatibility, an
entry for it needed to be added to the cmd_stub.

salt/fileserver/__init__.py

@@ -850,7 +850,8 @@ class FSChan(object):
                 self.opts['__fs_update'] = True
         else:
             self.fs.update()
-        self.cmd_stub = {'master_tops': {}}
+        self.cmd_stub = {'master_tops': {},
+                         'ext_nodes': {}}
 
     def send(self, load, tries=None, timeout=None, raw=False):  # pylint: disable=unused-argument
         '''