Re-integrate graph flow

[Joshua Harlow]

Currently the graph flow is broken by https://review.openstack.org/#/c/45139/ due to the unification of tasks and linear_flow under the new engine concept. We need to fix up the support for that (by having an graph pattern, likely pretty similar to the current graph pattern) and then having an action engine concepts (one that runs the graph serially)

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/45139
Committed: http://github.com/stackforge/taskflow/commit/23dfff410587d0f137bc59e10d953396802455d7
Submitter: Jenkins
Branch: master

commit 23dfff410587d0f137bc59e10d953396802455d7
Author: Joshua Harlow <email address hidden>
Date: Wed Sep 4 12:47:26 2013 -0700

    Engine, task, linear_flow unification

    In order to move away from the existing flows having their
    own implementation of running, start moving the existing
    flows to be patterns that only structure tasks (and impose
    constraints about how the group of tasks can run) in useful
    ways.

    Let the concept of running those patterns be handled by an
    engine instead of being handled by the flow itself. This
    will allow for varying engines to be able to run flows in
    whichever way the engine chooses (as long as the constraints
    set up by the flow are observed).

    Currently threaded flow and graph flow are broken by this
    commit, since they have not been converted to being a
    structure of tasks + constraints. The existing engine has
    not yet been modified to run those structures either, work
    is underway to remediate this.

    Part of: blueprint patterns-and-engines

    Followup bugs that must be addressed:
      Bug: 1221448
      Bug: 1221505

    Change-Id: I3a8b96179f336d1defe269728ebae0caa3d832d7