Enforce users to specify a 'pool' dimension when triggering a task.

client/example/2_swarming_run.py

 #!/usr/bin/env python
 # Copyright 2012 The Swarming Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0 that
 # can be found in the LICENSE file.
 
 """Runs hello_world.py, through hello_world.isolate, remotely on a Swarming
 bot.
 
 It first 'compiles' hello_world.isolate into hello_word.isolated, then requests
 via swarming.py to archives, run and collect results for this task.
@@ skipping 22 matching lines @@
         'Running the job remotely. This:\n'
         ' - archives to %s\n'
         ' - runs and collect results via %s' %
         (options.isolate_server, options.swarming))
     cmd = [
       'swarming.py',
       'run',
       '--swarming', options.swarming,
       '--isolate-server', options.isolate_server,
       '--dimension', 'os', options.swarming_os,
+      '--dimension', 'pool', 'default',
       '--task-name', options.task_name,
       '--task-summary-json', 'example_result.json',
       '--decorate',
       isolated,
     ]
     if options.idempotent:
       cmd.append('--idempotent')
     if options.priority is not None:
       cmd.extend(('--priority', str(options.priority)))
     common.run(cmd, options.verbose)
     with open('example_result.json', 'rb') as f:
       print('example_result.json content:')
       print(f.read())
     return 0
   except subprocess.CalledProcessError as e:
     return e.returncode
   finally:
     shutil.rmtree(tempdir)
 
 
 if __name__ == '__main__':
   sys.exit(main())