Ability to modify the actions workflow of an action plan

Following https://bugs.launchpad.net/watcher/+bug/1533281, we need to provide a mechanism in Watcher that would enable an administrator to *safely* modify the action workflow of an action plan.

Here below are some of the questions we need to answer:

- In what way should we be able to alter an action plan after it has been
  issued?
- What are the mechanisms we would have put in place to rebuild the flow of
  actions after it got updated?

  * What rules are we setting to apply to re-map an action that does not have
    a parent anymore but still has some children actions attached to it?
  * Should we then submit the new version of the action flow to the planner?