Refactor networking assets finder

[hakman]

Most notable changes are:

• extract the LyftVPC assets finder to separate function (similar to the CNI one)
• add test for the new function and update previous ones
• use SHA from repo for the CNI asset finder (similar to the kubelet one)
  

  kops/pkg/assets/builder.go

  Line 249 in 781279e

  // TODO - remove this method as CNI does now have a SHA file

Note: All file assets can be remapped using Spec.Assets.FileRepository, so maybe we should remove some of these env vars like CNI_VERSION_URL and LYFT_VPC_DOWNLOAD_URL .

[k8s-ci-robot]

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[hakman]

@rifelpet Spec.Assets.FileRepository could make offline usage much simpler, especially if Docker and Containerd packages can be moved here.

[johngmyers]

/lgtm