monitor: delegate argument checking to argparse

So instead of a traceback we get a better error message
written by argparse. Also , it will print a better help
message

slapos/monitor/runpromise.py

@@ -28,7 +28,7 @@ def getArgumentParser():
   parser = argparse.ArgumentParser()
   parser.add_argument('-c', '--config', dest='config_file',
                       help='The Path of configuration file to load.')
-  parser.add_argument('-p', '--partition-folder',
+  parser.add_argument('-p', '--partition-folder', required=True,
                       help='Base path of the partition.')
   parser.add_argument('-L', '--log-folder',
                       help='Folder where promises will write logs.')
@@ -124,9 +124,6 @@ class MonitorPromiseLauncher(object):
       with open(self.config.pid_path, 'w') as fpid:
         fpid.write(str(os.getpid()))
 
-    if not self.config.partition_folder:
-      raise ValueError("Partition folder is not specified")
-
     parameter_dict = {
       'promise-timeout': self.config.promise_timeout or promise_timeout,
       'promise-folder': os.path.join(self.config.partition_folder, 'etc', 'plugin'),