Convert allowInsecureRegistries to list of registries instead of binary true/false

[chanseokoh]

I think this is a nice enhancement and useful for multi-module projects. It ensures only those registries are insecure, preventing an issue like https://github.com/GoogleContainerTools/skaffold/pull/2674#issuecomment-523497039.

Potentially it can accept a glob pattern. Or, maybe just a suffix for simplicity.

We also need to think about aliases.

[briandealwis]

And since registries would be explicitly identified then we could automatically apply sendCredentialsOverHttp for those registries.

[chanseokoh]

Or perhaps automatically set sendCredentialsOverHttp only for loopback addresses (localhost). AFAIK, OS by-passes a network stack entirely for loopback as optimization, so it's pretty safe.

[JoeWang1127]

Closing as part of cleanup. Please re-open if it's still relevant