conflict alarm name when alarm-create fail with 400 instead of 409

[ZhiQiang Fan]

# ceilometer alarm-create --meter instance --threshold 10 --name quota-alarm
# ceilometer --debug alarm-create --meter instance --threshold 10 --name quota-alarm
HTTP/1.0 400 Bad Request
date: Wed, 22 Jan 2014 12:16:46 GMT
content-length: 107
content-type: application/json
server: WSGIServer/0.1 Python/2.7.3

{"error_message": {"debuginfo": null, "faultcode": "Client", "faultstring": "Alarm with that name exists"}}

WARNING (http:168) Request returned failure status.
HTTPBadRequest (HTTP 400)

see:
https://github.com/openstack/ceilometer/blob/master/ceilometer/api/controllers/v2.py#L1592
https://github.com/openstack/ceilometer/blob/master/ceilometer/api/controllers/v2.py#L72

there also is a 404 but return 500 for alarm not found:
https://github.com/openstack/ceilometer/blob/master/ceilometer/api/controllers/v2.py#L1294

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/68775
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=534608a53a7485e35ca7a2819cbaeb10894f05b4
Submitter: Jenkins
Branch: master

commit 534608a53a7485e35ca7a2819cbaeb10894f05b4
Author: ZhiQiang Fan <email address hidden>
Date: Thu Jan 23 17:36:38 2014 +0800

    Use explicit http error code for api v2

    * not exist should be 404
    * conflict should be 409
    * not authorized should be 401

    Change-Id: Ied88290c42e5679761e6d50073c7965c5644f84e
    Closes-Bug: #1271525