enable configurable rate-limiting for PyPI

Configured in pypi pillar data with:

    rate_limit:
      enable: True
      zone_size: 30m
      max_rate: 5r/s
      burst: 10

only the toplevel key, and enable are required

zone_size, max_rate, and burst are optional
add missing template
add configuration for nodelay
now that we have an nginx taht supports it, set limit_req_status to 429
Merge pull request #27 from python/rate-limiting

enable configurable rate-limiting for PyPI