Switch to new update URIs

[tjkirch]

Description of changes:

This uses the new OS information made available in #777 to switch us to our new updates URIs that include variant and arch.

Testing done:

Basic tests done with #777.

The new URI values show up:

  "updates": {
    "metadata-base-url": "https://updates.bottlerocket.aws/2020-02-20/aws-k8s/x86_64/",
    "target-base-url": "https://updates.bottlerocket.aws/targets/",

[zmrow]

🍀

[tjkirch]

This push changes the date to 2020-02-02 per discussion above :P

[iliana]

I disagree with the date change but am not going to block on it. :)

🚢

[tjkirch]

This push rebases on changes from #777.

[tjkirch]

Just a rebase on develop to fix the conflict with the motd setting.