unit tests fail if https_proxy is set

https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-artful/artful/amd64/p/python-boto/20170531_165016_ad9d3@/log.gz

Python-boto unit tests don't like *_proxy env vars set during execution. They do not reach out to the network, but different codepaths are taken when a proxy is detected, and the mocks get destroyed in that case.

A lot of failures of the form:

======================================================================
FAIL: test_head_bucket_other (tests.unit.s3.test_connection.TestHeadBucket)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/tmp/autopkgtest.JJMfC2/autopkgtest_tmp/tests/unit/s3/test_connection.py", line 331, in test_head_bucket_other
    self.assertEqual(err.status, 405)
AssertionError: 403 != 405
-------------------- >> begin captured logging << --------------------
boto: DEBUG: Using access key provided by client.
boto: DEBUG: Using secret key provided by client.
boto: DEBUG: path=/
boto: DEBUG: auth_path=/you-broke-it/
boto: DEBUG: Method: HEAD
boto: DEBUG: Path: https://you-broke-it.s3.amazonaws.com/
boto: DEBUG: Data:
boto: DEBUG: Headers: {}
boto: DEBUG: Host: you-broke-it.s3.amazonaws.com
boto: DEBUG: Port: 443
boto: DEBUG: Params: {}
boto: DEBUG: establishing HTTPS connection: host=you-broke-it.s3.amazonaws.com, kwargs={'port': 443, 'timeout': 70}
boto: DEBUG: Proxy connection: CONNECT you-broke-it.s3.amazonaws.com:443 HTTP/1.0

boto: DEBUG: Token: None
boto: DEBUG: StringToSign:
HEAD

Wed, 31 May 2017 16:49:14 GMT
/you-broke-it/
boto: DEBUG: Signature:
AWS aws_access_key_id:Sy9u+sHaYcVAlgRndJo4iOD3mCY=
boto: DEBUG: Final headers: {'Date': 'Wed, 31 May 2017 16:49:14 GMT', 'Content-Length': '0', 'Authorization': u'AWS aws_access_key_id:Sy9u+sHaYcVAlgRndJo4iOD3mCY=', 'User-Agent': 'Boto/2.44.0 Python/2.7.13 Linux/4.10.0-21-generic'}
boto: DEBUG: Response headers: [('x-amz-id-2', 'R8k7qp47lyMqK4+O5bc5vLcFHbwHB64IifV0XEyEdJSNC7q1FGe3o/7LkUGjkR3eYBjDwyE8Oz8='), ('server', 'AmazonS3'), ('transfer-encoding', 'chunked'), ('x-amz-request-id', 'E92141CA21DA2EEC'), ('date', 'Wed, 31 May 2017 16:49:14 GMT'), ('content-type', 'application/xml')]
--------------------- >> end captured logging << ---------------------