Implement parallel delete

Implement parallel delete

This is first patch in the series to implement parallel delete
and it includes following changes:

1. changing Stack.delete to use stack_task
2. In Resource.delete polling for check_delete_complete
3. Updating unit tests to use TaskRunner for resource.delete and
   resource.destroy

Blueprint parallel-delete

Change-Id: I8a96b1a956388372c29f9f45445f397467667110