Add "keep-result" task option

We need to add option that would tell Mistral engine not to keep task result (actually its action execution results) after task completion. It's needed for the cases when we know that action result is large and it doesn't make sense to keep it as is. Instead, we may want to extract only a part of it using "publish" clause.

tasks:
  my_task:
    action: nova.servers_list
    publish:
      vm_count: <% $.my_task.length() %>
    keep-result: false