client HTTPClient __init__ fails if auth_url None

The initializer for the python-cinderclient HTTPClient class
fails if passed a None for an auth_url.
This patch modifies the inializer to not call rstrip
if auth_url is None, matching the initalizers in
python-novaclient and python-neutronclient

Change-Id: I19dd6911816639a0e0d6175ba910e9777a4b5981
Closes-bug: #1358926
Merge "client HTTPClient __init__ fails if auth_url None"