Simplify file writing.

slapos/grid/SlapObject.py

@@ -405,9 +405,7 @@ class Partition(object):
     parameter_dict = self.computer_partition.getInstanceParameterDict()
     if 'timestamp' in parameter_dict:
       timestamp_path = os.path.join(self.instance_path, '.timestamp')
-      descriptor = open(timestamp_path, 'w')
-      descriptor.write(parameter_dict['timestamp'])
-      descriptor.close()
+      open(timestamp_path, 'w').write(parameter_dict['timestamp'])
 
   def start(self):
     """Asks supervisord to start the instance. If this instance is not