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

client/example/4_swarming_run_manual_upload.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 a manually crafted hello_world.isolated, remotely
 on a Swarming slave.
 
 No .isolate file is involved at all.
 
@@ skipping 55 matching lines @@
     # Now trigger as usual. You could look at run_exmaple_swarming_involved for
     # the involved way but use the short way here.
 
     common.note('Running %s on %s' % (isolated_sha1, 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,
       isolated_sha1,
     ]
     if options.idempotent:
       cmd.append('--idempotent')
     if options.priority is not None:
       cmd.extend(('--priority', str(options.priority)))
     common.run(cmd, options.verbose)
     return 0
   except subprocess.CalledProcessError as e:
     return e.returncode
 
 
 if __name__ == '__main__':
   sys.exit(main())