Missing buffer size option for wsgi lead to 502

When deploying a Flask app to EBS (using the [tutorial](http://docs.aws.amazon.com/elasticbeanstalk/latest/dg/create-deploy-python-flask.html)) one can face 502 HTTP status code when returning response with a size bigger than 4096.
```
invalid request block size: 4654 (max 4096)...skip
```

Apparently this script does not contemplate to configure the buffer-size option for WSGI.
After reading the doc about [configuration on EBS](http://docs.aws.amazon.com/elasticbeanstalk/latest/dg/create-deploy-python-container.html#create-deploy-python-custom-container-envprop) I am not sure if I can specify such an option without doing a hacky replace of the `uwsgi-start.sh` script and rerun it.

Start by reading the uwsgi-start.sh script and the linked Elastic Beanstalk Python container configuration documentation. Reproduce the 502 with a response larger than 4096 and trace how WSGI options are passed. Done means the buffer-size option can be configured through the supported EBS setup without replacing the script.

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