Convert scripts/install-cni.sh to a golang

[jingax10]

It is hard to unit test CNI generator by scripts/install-cni.sh. We want to convert to golang to make it unit testable.

[rlenglet]

@varunmar Can you please reassign this issue to @bcheung? He who will start working on it soon.

[varunmar]

Hmm. I think I can't assign it to him directly. I wonder if it restricts it to the GoogleCloudPlatform org or something

[jingyuanliang]

CNI writer is there now.