Add a trust notifier task

[OpenStack Infra]

https://review.openstack.org/329520
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/zaqar" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit 51604b4954b14a2ae14b15b50c302d32db0e40a7
Author: Thomas Herve <email address hidden>
Date: Wed Jun 8 16:06:28 2016 +0200

    Add a trust notifier task

    This adds the ability to send keystone authentified notifications using
    trusts. To do so, you specify the posted URL with the "trust+" prefix,
    and Zaqar will create and store a trust when subscribing to a queue, if
    the trust not provided in the subscription options

    It also add a capability to the webhook task to be able to send more
    structured data in the notification, allowing to include the Zaqar
    message in the data.

    blueprint mistral-notifications
    DocImpact
    Change-Id: I12b9c1b34cdd220fcf1bdc2720043d4a8f75dc85