Use wsgi.response for v2.1 API

Now a decorator @wsgi.response() can be used for specifying success
status code. Most v2.1 APIs use the decorator and it makes the code
more readable. This patch replaces webob.Response with the decorator
for the other APIs.
In addition, this patch changes some orders of decorators for showing
success code and expected error codes clearly.

Even if applying this patch, some webob.Response() calls still exist
under the path nova/api/openstack/compute/plugins/v3/ because these
cases need to contain some information in a response header.

Partially implements blueprint v2-on-v3-api

Change-Id: I419819b8e55427979e8d82b972cd237952a5b06b
Merge "Use wsgi.response for v2.1 API"