Use httplib constants for http status codes
Use httplib constants for http status codes

The httplib (py2) and http.client (py3) have nice constants for all
the standard HTTP status codes. This patch changes the exception
module to make use of those.

Change-Id: I67d3717dca5aecf3c8b36f5b69f1ca659c0b08b1

openstack/keystone,openstack/keystone,ilay09/keystone,rajalokan/keystone,ilay09/keystone,openstack/keystone,rajalokan/keystone,rajalokan/keystone,mahak/keystone,ilay09/keystone,mahak/keystone,mahak/keystone