Model the model

[tjkirch]

commit 9c7181a2ac01de120b6da826dc1594565508c835
Author: Tom Kirchner <tjk@amazon.com>
Date:   Tue Mar 3 11:38:37 2020 -0800

Add a top-level model struct and API route

Several benefits here:
* No longer need erased-serde or HashMaps to represent template data
* One API call to see everything the API system knows
  * Useful for curious humans
  * Useful as data source for templating

This also leads us toward the ability to have entirely different models for
different variants, though the API server doesn't support that yet.

commit 3980e543e206e50adadd2cebae3be3a580a23809
Author: Tom Kirchner <tjk@amazon.com>
Date:   Thu Mar 5 13:45:25 2020 -0800

common_migrations: Remove HashMap hack using local struct

This was found when adding a top-level Model struct, which the old comment said
would help with this case, but it actually didn't because we can't use the
model in migrations.  (Would you use the old or the new?)

Instead, we can remove the HashMap by making a single-use struct with a
'settings' key.  This makes it more obvious that only "settings" keys are
supported, so make the docstring reflect that reality too.

---

🐈 🚶‍♂️

One potential downside that's at least worth considering - this may give the appearance that you can request any given key from the apiserver; since now we have "/" and "/settings" you may expect you can get the tree at "/settings/updates". We couldn't do that without a fancier apiserver.

Testing done:

Launched an instance, it joined my cluster and ran a pod, was running, seemed OK. I used apiclient and saw that the /settings and /os APIs still work, and saw the new top-level API return everything:

{
  "settings": {
    "motd": "Welcome to Bottlerocket!",
...
  "services": {
    "kubernetes": {
      "configuration-files": [
        "kubelet-env",
...
  "configuration-files": {
    "chrony-conf": {
      "path": "/etc/chrony.conf",
...
  "os": {
    "pretty_name": "Bottlerocket OS 0.3.0",
...

[etungsten]

I was going to ask if you could test the migration helper but then realized that we've removed all migrations. The changes look right and make sense.

[zmrow]

I like this change!

🎉