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