Fixes #9508 - use Dynflow sub-plans support for sub tasks and bulk actions

[iNecas]

Bulk actions is just one of the use-cases for using sub-plans. This patch
extract the common code to be used for triggering external tasks in
general and moves that to the Dynflow.

The usage is as follows: one needs to inherit it's action from the
Actions::ActionWithSubPlans and override the
create_sub_plans method. The create_sub_plans will be executed in
run phase and the rest of the action will make sure that the execution of
the action will wait till the sub plan is done. It also tracks the relation
between parent and child tasks, as it already did in bulk actions.

An example code could look like this (taken from real Katello actions):

class SyncAsSubPlan < Actions::ActionWithSubPlans
 input_format do
   param :id, Integer
 end

  def plan(repository)
   plan_self(:id => repository.id)
 end

  def create_sub_plans
   trigger(Repository::Sync, ::Katello::Repository.find(input[:id]))
 end
end

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[iNecas]

Depends on Dynflow/dynflow#145

[bbuckingham]

Ran a test of fusor which uses ActionWithSubPlans and it ran cleanly, with no unexpected behaviors. This included both this PR and the referenced dynflow PR.

[pitr-ch]

If you would like to make things more complicated [self, *(parent_task.with_parents if parent_task)] ;)

[pitr-ch]

1 noop and 1 minor, otherwise ACK

[iNecas]

Comments addressed. Waiting for the CI, then merging and releasing new version of foreman-tasks

[iNecas]

@bbuckingham foreman-tasks 0.6.13 and dynflow 0.7.7 containt this change.The PR for foreman-packaging is here theforeman/foreman-packaging#582

[bbuckingham]

Thanks @iNecas !