Using correct attribute of exception

Currently patch (https://review.openstack.org/#/c/69837/6) was merged in
python-novaclient. New exceptions have not attribute 'code' and now we
should use http_status instead, falling back to the old attribute if
the new attribute does not exist.

Change-Id: I16e54c8e19dd93229fd7bcfe124fd4f2595f4e59
Closes-bug: #1319499
Merge "Using correct attribute of exception"