Update Bundle download function

[yiheng-wang-nv]

The bundle hosting place has been defined, thus the download function should be updated.

[Nic-Ma]

Hi @yiheng-wang-nv ,

When our model-zoo repo is ready, please update the download command to use it as the default repo arg:
https://github.com/Project-MONAI/MONAI/blob/dev/monai/bundle/scripts.py#L154

Thanks.