title: tensorboard logdir with s3 path
body: <p>I see tensorflow support AWS s3 file system (<a href="https://github.com/tensorflow/tensorflow/tree/master/tensorflow/core/platform/s3" rel="nofollow noreferrer">https://github.com/tensorflow/tensorflow/tree/master/tensorflow/core/platform/s3</a>) but I am unable to use the S3 path with tensorboard.</p>
  
  <p>I tried latest nightly <code>0.4.0rc3</code> but no luck. I built locally also and made sure <code>Do you wish to build TensorFlow with Amazon S3 File System support? [Y/n]:</code> set to <code>YES</code> but still i don't see <code>tensorboard --logdir=s3://bucket/path</code> working at all.</p>
  
  <p>Am I missing something here?</p>