Rename client.py to httpclient.py

The discoverable entry point is to be client.Client however adding this
functionality to the current client.py is impossible as we end up with
circular dependencies.

This patch simply renames the current client.py to httpclient.py to make
future patches that will modify client.py more readable.

Required for: blueprint api-version-discovery

Change-Id: Ibcea03f6e1df0ae05329297166a8b8117fc3ce7b
Merge "Rename client.py to httpclient.py"