Fix test_misc for WebOb 1.5

Fix test_misc for WebOb 1.5

WebOb 1.5 was released at 2015-10-11. With this new version,
webob.exc.WSGIHTTPException() constructor now fails with a KeyError
when the HTTP status code is 0.

test_exceptions_raise() of test_misc tries to instanciate all
exceptions of cinder.exception. The problem is that
ConvertedException uses a default HTTP status code of 0.

Modify the default HTTP status code of ConvertedException to 400 to
fix the unit test. The bug is only in the test,
cinder/api/openstack/wsgi.py copies an existing HTTP code:

    Fault(exception.ConvertedException(code=ex_value.code,  ...)

Closes-Bug: #1505153
Change-Id: I1aec8038774828d48da4b0e831b390e33243809a