Use HTTPretty in S3 test code

Remove the httplib specific mocks that are used within the S3
middleware. This won't work when we convert S3 middleware to use the
requests library.

Change-Id: Iee5f3089286c980bef3a19dc9d5068399946e217
Related-Bug: #1275598
Python 3: make tests from v2_0/test_access.py pass

This fixes calls to the hash_signed_token() and cms_hash_token() functions, by
making sure they are given bytes.

Change-Id: I83ac48a845cd09150b01afad6f0549ee83c20ddd
Use requests library in S3 middleware

Convert the S3 token middleware to use the requests library. It does not
validate certificates as part of this patch to keep new functionality
isolated.

Change-Id: I0f26c80a969b919de80410af0a80920bd8493191
Closes-Bug: #1275598
Merge "Python 3: make tests from v2_0/test_access.py pass"
Merge "Use requests library in S3 middleware"