System Proxy Settings return a string instead of an integer for port

[sethrosenblum]

When we pull system proxy settings from urllib we end up with this error:

Traceback (most recent call last):
  File "transaction.pyc", line 190, in flush_next
  File "ddagent.pyc", line 272, in flush
  File "tornado\httpclient.pyc", line 206, in fetch
  File "tornado\curl_httpclient.pyc", line 95, in fetch_impl
  File "tornado\curl_httpclient.pyc", line 233, in _process_queue
  File "tornado\curl_httpclient.pyc", line 358, in _curl_setup_request
TypeError: strings are not supported for this option

this is because the proxy port is being set to a string, which tornado doesn't accept:

https://github.com/DataDog/dd-agent/blob/master/config.py#L606-L608