[doc] Add a generic and extensible way to describe the flow of actions

https://review.openstack.org/265185
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commit 86d3c2ff897d755070fc89efbd04afe0d8a7b556
Author: Jean-Emile DARTOIS <email address hidden>
Date: Fri Jan 8 12:03:55 2016 +0100

    Add a generic and extensible way to describe the flow of actions

    In watcher, an audit generates a set of actions which
    aims at achieving a given goal (lower energy consumption, ...).
    It is possible to configure different strategies in order to achieve
    each goal. Each strategy is written as a Python class which produces
    a set of actions. Today, the set of possible actions is fixed for a
    given version of Watcher and enables optimization algorithms to
    include actions such as instance migration, changing hypervisor state,
    changing power state (ACPI level, ...).

    This patchset propose a generic and extensible way to describe
    the actions and his parameters that we want to add to Action Plan.
    It also remove the static actions because they are now deprecated.

    The documentation regarding strategy plugin need to be
    updated (plugins.rst).

    DocImpact
    Partially implements: blueprint watcher-add-actions-via-conf

    Change-Id: I3d641080e8ad89786abca79a942c8deb2d53355b