Add manual dependency assignment

[jlsherrill]

It would be very nice to be able to manually assign dependencies when defining sub-actions of an action. Right now we can only rely on output of a previous action to set the dependency chain. This would allow us to do something where if the first actions failed (and we allow it to be skipped), the rest would abort as well.

Something like (as an example):

def plan(repo)         
  action_subject(repo)
  sequence do
    other_actions = []
    sync_task = plan_action(Pulp::Repository::Sync, pulp_id: repo.pulp_id)

    concurrence do
      other_actions << plan_action(Katello::Repository::NodeMetadataGenerate, repo, sync_task.output[:pulp_tasks])

      other_actions << plan_action(ElasticSearch::Repository::IndexContent, pulp_task: sync_task.output[:pulp_tasks], id: repo.id)
    end

    other_actions.each{|action| action.add_dependency(sync_task)}
  end
end