[windows][requests] Bundle default CA certs of requests

[olivielpeau]

py2exe only includes the python files of the python packages in the
library zipped file. This causes the requests library to be shipped
without CA certs, so all the https requests made with it fail (unless
SSL cert validation is explicitely disabled).

This fixes the issue by:

1. shipping the default certificates of requests with the Windows
   agent as an extra data file
2. overriding the path to the default CA bundle that requests uses
   (which is done in quite an ugly way, but I haven't found any better
   way)

Notes:

• we already ship CA certs for tornado, but to be as close as possible to the agent's behavior on other platforms I've used the default CA certs provided by requests/certifi
• this doesn't fix SSL cert validation on the http_check on Windows, because the check uses default paths that are only valid on unix platforms: see

  dd-agent/checks.d/http_check.py

  Lines 127 to 131 in 8fed4c9

  	CA_CERTS = [
  	'/opt/datadog-agent/embedded/ssl/certs/cacert.pem',
  	os.path.join(os.path.dirname(tornado.__file__), 'ca-certificates.crt'),
  	'/etc/ssl/certs/ca-certificates.crt',
  	]

  (unless the user sets a valid path in the check's config)

[remh]

Thanks for doing this!