Add offline apt package cache in kube runtime

[hzy46]

To resolve: #4211

Docs: https://github.com/microsoft/pai/blob/zhiyuhe/package_cache/src/kube-runtime/package_cache.md

[Binyang2014]

Currently it looks good to me.

For a long term, I think this feature maybe used by plugin developer and cluster admin
For plugin developer, it's better to describe its' desired packages under the plugin folder.
For cluster admin he/she only need to declare some packages need to be cached and it can be made as runtime build config

[hzy46]

Test cases:

1. submit a job using ubuntu:16.04; enable ssh and a storage for it; make sure SSH and storage is usable; make sure there is no apt-get command in generated logs

2. submit a job using ubuntu:15.10; enable ssh and a storage for it; make sure SSH and storage is usable; In this case, package cache should fall back to apt-get

[Binyang2014]

Test cases:

1. submit a job using ubuntu:16.04; enable ssh and a storage for it; make sure SSH and storage is usable; make sure there is no apt-get command in generated logs
2. submit a job using ubuntu:15.10; enable ssh and a storage for it; make sure SSH and storage is usable; In this case, package cache should fall back to apt-get

Test finished