Add diagrams to the architecture doc page

[Vincent Mahe]

Today in the page that describes the Watcher architecture, we can only find a diagram showing the organic view of the system.

We should also add the following diagrams to provide more information about the dynamic view of Watcher :
- sequence diagrams showing what happens in Watcher for the 3 main scenarios (Create Audit Template, Launch Audit, Launch Action Plan)
- state machine diagrams that show how Audit and Action Plans state can change and what makes them change

And also some diagrams showing the data model of Watcher, especially the functional dependency of objects from the actors point of view (Goals, Audits, Action Plans, ...).

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/266461

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/266461
Committed: https://git.openstack.org/cgit/openstack/watcher/commit/?id=8f87699910bb55aa9bb90238868ef19cd756d8c1
Submitter: Jenkins
Branch: master

commit 8f87699910bb55aa9bb90238868ef19cd756d8c1
Author: vmahe <email address hidden>
Date: Tue Jan 12 17:13:02 2016 +0100

    Add diagrams to the architecture doc page

    Added some data model diagrams, sequence diagrams and state machine
    diagrams.
    The state machine diagrams and sequence diagrams are built with
    PlantUML whereas data model diagrams are built with Dia.
    Also added some textual description with the sequence diagrams.

    Change-Id: Iffbb47b0f2d12ce63eeaa1531a1bd1a790d69e79
    Closes-Bug: #1531802