Mistral API returns error with code 500 when user creates 2 workbook with the one name

[Timur Nurlygayanov]

Steps To Reproduce:
1. Send request to create workbook 'test'.
2 .Send request to create workbook 'test' again.

Python Tests Example:
from mistralclient.api import client

c = client.Client(username='admin', api_key='swordfish',
                  project_name='admin', auth_url='http://172.18.124.203:5000/v2.0/',
                  mistral_url='http://localhost:8989/v1')

c.workbooks.create('test')
c.workbooks.create('test')

Observed Result:
Error message:

Traceback (most recent call last):

  File "/home/ubuntu/mistral/.tox/venv/local/lib/python2.7/site-packages/wsmeext/pecan.py", line 77, in callfunction
    result = f(self, *args, **kwargs)

  File "/home/ubuntu/mistral/mistral/api/controllers/v1/workbook.py", line 75, in post
    wb = workbooks.create_workbook(workbook.to_dict())

  File "/home/ubuntu/mistral/mistral/services/workbooks.py", line 21, in create_workbook
    workbook = db_api.workbook_create(values)

  File "/home/ubuntu/mistral/mistral/db/api.py", line 66, in workbook_create
    return IMPL.workbook_create(values)

  File "/home/ubuntu/mistral/mistral/db/sqlalchemy/api.py", line 63, in decorator
    res = func(*args, **kwargs)

  File "/home/ubuntu/mistral/mistral/db/sqlalchemy/api.py", line 108, in _within_session
    result = func(*args, **kw)

  File "/home/ubuntu/mistral/mistral/db/sqlalchemy/api.py", line 267, in workbook_create
    raise Exception

Exception

[Timur Nurlygayanov (tnurlygayanov)]

The fix proposed in https://review.openstack.org/#/c/64107/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/64107
Committed: https://git.openstack.org/cgit/stackforge/mistral/commit/?id=2176d57d15ac6b2090a2c953005749f8432e1bc5
Submitter: Jenkins
Branch: master

commit 2176d57d15ac6b2090a2c953005749f8432e1bc5
Author: Nikolay Mahotkin <email address hidden>
Date: Thu Dec 26 13:13:53 2013 +0400

    Fix workbook POST duplicate exception

     * 400 status code response on duplicate error
     * new exceptions - Mistral, DBDuplicateEntry
     * import exception from openstack-common

    Fixes bug: #1263942

    Change-Id: Ia802db5c6b30be96cb0b8ce2cfc0f7fd04122302