fix KeyError when using redirect_x in CommandLineInterface

Author: oesteban

nipype/interfaces/base.py

@@ -987,7 +987,8 @@ def run(self, **inputs):
                     time.sleep(0.2)  # give Xvfb time to start
                     if xvfb_proc.poll() is not None:
                         raise Exception('Error: Xvfb did not start')
-                    old_displaynum = os.environ['DISPLAY']
+
+                    # old_displaynum = os.getenv('DISPLAY')
                     runtime.environ['DISPLAY'] = ':%s' % vdisplay_num
 
             runtime = self._run_interface(runtime)