title: gcloud info --format json doesn't work
body: <p>When running <code>gcloud info --format json</code> on a GCE instance I get the following exception : </p>
  
  <pre><code>% gcloud info --format json
  Traceback (most recent call last):
    File "/usr/local/bin/../share/google/google-cloud-sdk/./lib/googlecloudsdk/gcloud/gcloud.py", line 175, in &lt;module&gt;
      main()
    File "/usr/local/bin/../share/google/google-cloud-sdk/./lib/googlecloudsdk/gcloud/gcloud.py", line 171, in main
      _cli.Execute()
    File "/usr/local/bin/../share/google/google-cloud-sdk/./lib/googlecloudsdk/calliope/cli.py", line 385, in Execute
      post_run_hooks=self.__post_run_hooks, kwargs=kwargs)
    File "/usr/local/bin/../share/google/google-cloud-sdk/./lib/googlecloudsdk/calliope/frontend.py", line 274, in _Execute
      pre_run_hooks=pre_run_hooks, post_run_hooks=post_run_hooks)
    File "/usr/local/bin/../share/google/google-cloud-sdk/./lib/googlecloudsdk/calliope/backend.py", line 930, in Run
      output_formatter(result)
    File "/usr/local/bin/../share/google/google-cloud-sdk/./lib/googlecloudsdk/calliope/backend.py", line 922, in Format
      resource_printer.Print(obj, args.format or 'yaml', out=log.out)
    File "/usr/local/bin/../share/google/google-cloud-sdk/./lib/googlecloudsdk/core/util/resource_printer.py", line 421, in Print
      formatter.PrintSingleRecord(resources)
    File "/usr/local/bin/../share/google/google-cloud-sdk/./lib/googlecloudsdk/core/util/resource_printer.py", line 88, in PrintSingleRecord
      separators=(',', ': '))
    File "/home/Tradelab/anaconda/lib/python2.7/json/__init__.py", line 189, in dump
      for chunk in iterable:
    File "/home/Tradelab/anaconda/lib/python2.7/json/encoder.py", line 442, in _iterencode
      o = _default(o)
    File "/home/Tradelab/anaconda/lib/python2.7/json/encoder.py", line 184, in default
      raise TypeError(repr(o) + " is not JSON serializable")
  TypeError: &lt;__calliope__command__.93a535615504421c846125f4c0a6cb7e.gcloud.info.InfoHolder object at 0x7f70752ad950&gt; is not JSON serializable
  </code></pre>
  
  <p>I can avoid this by editing <code>resource-printer.py</code> but I am looking for a cleaner solution. </p>