Add an example of using v3 client with sessions

Added a bit more information to the 'using-api-v3' section, about
using sessions instead of passing in arguments.
This overlaps with what is documented in 'using-sessions' but I
think it's still handy as most folks will likely go here first.

Change-Id: I352bda49e01362d3a5da9a79ceb34748552ef9ec
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
Merge "Add an example of using v3 client with sessions"