[FIX] MapNodes failed when MultiProcPlugin passed by instance

When calling ``workflow.run(plugin=MultiProcPlugin())``, MapNodes are set the ``use_plugin`` member that is not pickleable if ``plugin`` is a ``MultiProcPlugin`` object.

Author: oesteban

nipype/pipeline/engine/workflows.py

@@ -566,6 +566,8 @@ def run(self, plugin=None, plugin_args=None, updatehash=False):
             plugin = config.get('execution', 'plugin')
         if not isinstance(plugin, (str, bytes)):
             runner = plugin
+            plugin = runner.__class__.__name__[:-len('Plugin')]
+            plugin_args = runner.plugin_args
         else:
             name = '.'.join(__name__.split('.')[:-2] + ['plugins'])
             try: