Convert httpretty to requests-mock

HTTPretty was great particularly when we were having to deal with both
httplib and requests together. Since that time the library itself has
caused a number of problems including backwards incompatible changes,
broken releases and some dependency issues that make it difficult to
package. There are also issues with memcache tests or anything else
that also tries to use the socket.

requests-mock does a very similar job, with a very similar interface
however it targets only requests and so doesn't have the same socket
issues and will be a much easier dependency on packagers.

keystoneclient is the first of a number of clients that will do the
changeover.

Change-Id: Ida6e5feb71b6ff6662fb24b9fa6535b039c99d96