Convert to requests-mock

We've had some trouble with httpretty in the past and so are moving to
requests-mock. There should be no functionality change in this patch,
simply a transition to a newer library.

Examples: 
 - Python 2/3 inconsistencies
 - Breaking compatibility between releases
 - Incorrect package dependency specifications
 - Problems with distro packaging around tests 
 - *can* introduce a maintained state between tests.

Change-Id: I666a5c7e6747f0c5c2dc96336774fd0fcd3f5907
Merge "Convert to requests-mock"