Simple mirror support

[justinsb]

We recognize our primary location via string-matching, and we then
have a hard-coded list of mirrors for that location.

Didn't prove easy to make this much better, but we can hopefully do so
iteratively (e.g. fetch mirrors via URL)

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[justinsb]

Two things of note:

• nodeup will try all the mirrors, and so the fact that not all the artifacts are present for the older github releases won't matter (one http 404). I'm not planning on backporting, as that would mean going back and adding binaries to the old releases.
• the current mirrors won't do much - github and an S3 bucket, but we're hopefully getting a CNCF bucket soon. Maybe by the 1.12 release, but then we can just throw it onto the pile!

[zetaab]

/lgtm

I am thinking could we use cloudfront in front of normal s3 bucket? cloudfront will cache things to different regions. Maybe that is something that we could think of doing when we have cncf bucket?