Update Orchestration authorization model in Administrator Guide

[Kaz Shinohara]

Current `Orchestration authorization model` does not refer about `reauthentication_auth_method` configuration option, better to add the description about this so that every admin user will be able to get further understanding how to handle long running tasks by Heat.
https://github.com/openstack/heat/blob/master/heat/common/config.py#L106

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-----------------------------------
Release: 15.0.0 on 2017-05-18 20:25
SHA: 901ce995b3cc4f407cf7b09a9e3152554e993e1a
Source: https://git.openstack.org/cgit/openstack/openstack-manuals/tree/doc/admin-guide/source/orchestration-auth-model.rst
URL: https://docs.openstack.org/admin-guide/orchestration-auth-model.html

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/470795

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/470795
Committed: https://git.openstack.org/cgit/openstack/openstack-manuals/commit/?id=d71cec2d70c2a291ac6684d16476f1732b6d2e2f
Submitter: Jenkins
Branch: master

commit d71cec2d70c2a291ac6684d16476f1732b6d2e2f
Author: kaz_shinohara <email address hidden>
Date: Mon Jun 5 10:35:24 2017 +0900

    Added description about `reauthentication_auth_method`.

    The current Administrator Guide does not refer about
    `reauthentication_auth_method` in `heat.conf`, which is mandatory parameter
    to fully enable the trust authorization model for heat.

    Change-Id: I5eb10c1f257f52b44e9c75b7bf2f65ac52ce9f98
    Closes-Bug: #1691966