Fix incorrect kwargs 'reason' for HTTPBadRequest

webob.exc.HTTPBadRequest doesn't have attribute 'reason', and its
__init__ method doesn't have such parameter too, so if we use such
kwargs, it will not be accepted by constructor of base class
WSGIHTTPException, then the http response body will format message
using default explanation instead of what we expected.

We should use kwargs 'explanation' instead of wrong 'reason'.

Change-Id: I02619164176a723e313ae274e527948d45fa6cad
Closes-Bug: #1264223
Merge "Fix incorrect kwargs 'reason' for HTTPBadRequest"