Add support for --override-platform flag

[rittneje]

Closes #2118.

[mtrmac]

Thanks!

My immediate reaction is that I don’t know how I feel about the way platforms.Parse defaults values; compare #1987 (comment) . The trade-off is consistency with containerd, which is certainly somewhat valuable.

Either way, this and #1987 should probably end up with a consistent answer to this trade-off.

[rhatdan]

@nalind thoughts?

[nalind]

This doesn't break anything, and it gets us closer to not being in the business of figuring out for ourselves which "variant" value to use on ARM boxes, so I'm fine with it. If that means reworking #1987, or scrapping it entirely, I'm not going to be particularly upset.