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
51604b4954