Port to python-requests

Currently, httplib implementation does not support SSL certificate
verification. This patch fixes this. Note that ssl compression parameter
and 100-continue thing is still missing from requests, though those are
lower priority.

Requests now takes care of:
* proxy configuration (get_environ_proxies),
* chunked encoding (with data generator),
* bulk uploading (with files dictionary),
* SSL certificate verification (with 'insecure' and 'cacert' parameter).

This patch have been tested with requests 1.1.0 (CentOS 6) and requests
2.2.1 (current version).

Change-Id: Ib5de962f4102d57c71ad85fd81a615362ef175dc
Closes-Bug: #1199783
DocImpact
SecurityImpact
Merge "Port to python-requests"