Can not get cron trigger instance by id

[int32bit]

Currently, we can only get/delete a trigger by its name via API, and also we can't create a cron trigger instance with same name even though in different tenant. I think it may be not a good practice to use `name` as uniq field, I think use `id` is better.

[Nikolay Makhotkin (nmakhotkin)]

Related change: https://review.openstack.org/#/c/496157/

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/496157
Committed: https://git.openstack.org/cgit/openstack/mistral/commit/?id=f84952a3b13d8b4a3eca07c558e7bed7378ebd62
Submitter: Jenkins
Branch: master

commit f84952a3b13d8b4a3eca07c558e7bed7378ebd62
Author: int32bit <email address hidden>
Date: Tue Aug 22 16:36:38 2017 +0800

    Add get cron-trigger by id support

    Currently we can only do a CURD action on a cron-trigger by name.
    This patch refer to workflow implementation and re-encapsulate
    DB API so that users can manage a cron-trigger by id or name.

    Closes-Bug: 1684469

    Change-Id: I9ff657b2604647e734b5539e9bd6a524a3a20efb

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/mistral 6.0.0.0b1 development milestone.