Add /os API

[tjkirch]

commit 57daab4036dfb27618406ea2b1a52359b10fdb79
Author: Tom Kirchner <tjk@amazon.com>
Date:   Tue Feb 25 15:17:16 2020 -0800

Allow use of OS information in schnauzer templates

commit 0f9ab5d516084ae860c4b727c88e98e8d8414557
Author: Tom Kirchner <tjk@amazon.com>
Date:   Tue Feb 25 15:15:26 2020 -0800

Add /os to API to describe version, variant, arch, etc.

commit 65929d34de319236afc21e328c88ddbf47129644
Author: Tom Kirchner <tjk@amazon.com>
Date:   Tue Feb 25 15:12:15 2020 -0800

Add architecture to BottlerocketRelease

---

Testing done:

Unit tests still pass and a pod runs OK.

The new API:

[ec2-user@ip-192-168-107-15 ~]$ apiclient -u /os
{"pretty_name":"Bottlerocket OS 0.2.2","variant_id":"aws-k8s","version_id":"0.2.2","build_id":"d4207464-dirty","arch":"x86_64"}

I tested the schnauzer changes by updating our updates URIs to use the OS information, which is shown in #778.

[zmrow]

Other than pointing to /usr/lib/os-release as @bcressey / @iliana mentioned, LGTM.

😋

[tjkirch]

This push adds a commit using the /usr/lib/os-release path.