it is bad to create a trust for every alarm creation

[Liusheng]

When we create a alarm with action with trust notifier, aodh will create trust, that may cause many created if we have many alarms, and mostly the trust parameters are same (trustor_user, trustee_user, project, role_names). it is better to reuse the trust created previously.

There is a obstacle to fix this issue. previously when delete the alarm we also delete the corresponding trust, if we reuse the trust, we need to delete the trust when last alarm with this trust is deleted.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/413321

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/413321
Committed: https://git.openstack.org/cgit/openstack/aodh/commit/?id=2d157539244193ab7cedd890460689ccb170df55
Submitter: Jenkins
Branch: master

commit 2d157539244193ab7cedd890460689ccb170df55
Author: Zane Bitter <email address hidden>
Date: Tue Dec 20 19:24:34 2016 -0500

    Don't create multiple trust IDs per alarm

    Previously, if an alarm had multiple trust+ actions then we would create
    a new trust ID for each one. We would also create new trusts for them
    all each time the alarm was updated (even for actions that hadn't
    changed).

    With this change, we will only use a single trust ID for each alarm that
    contains one or more trust+ actions.

    Change-Id: Ib556ed3248221d6efb36f2117991c47e665ce336
    Partial-Bug: #1575113

[Zane Bitter (zaneb)]

Another reason it's bad: you can't create a trust from a trust token. So if the alarm triggers some automated change that results in the alarm being replaced, also using a trust token, then the new alarm won't be able to create a trust. (With the partial fix above, the alarm can at least be updated in place with a trust token, which was not the case before.)