Allow overriding the "pretty" name of the OS inside the image

[tjkirch]

Issue number:

Fixes #1314 - thanks @diranged!

Description of changes:

Similar to #1312, this passes another field through for user customization - this time the "pretty name" that shows up in os-release, which in turn is used by k8s and other tools.

The second commit reorganizes buildsys's builder module to use argument vectors instead of strings, so we can handle arbitrary characters, rather than using "|" as a marker (and subsequently making it unavailable to names).

Testing done:

Specifying a pretty name works:

> cargo make -e BUILDSYS_PRETTY_NAME="My Pretty Name" && cargo make ami -e BUILDSYS_PRETTY_NAME="My Pretty Name"
...
bash-5.0# cat /etc/os-release
PRETTY_NAME="My Pretty Name 1.0.5"
...

Default is still OK:

> cargo make && cargo make ami
...
bash-5.0# cat /etc/os-release 
PRETTY_NAME="Bottlerocket OS 1.0.5"
...

Retested with the second (refactor) commit and found that buildsys still produced the artifacts in the same way, and the resulting AMIs were still happy and healthy. Here's one with a weird name:

> cargo make -e BUILDSYS_PRETTY_NAME="My|Pretty@Name" && cargo make ami -e BUILDSYS_PRETTY_NAME="My|Pretty@Name"
...
bash-5.0# cat /etc/os-release 
PRETTY_NAME="My|Pretty@Name 1.0.5"
...
bash-5.0# apiclient -u /os      
{"pretty_name":"My|Pretty@Name 1.0.5","variant_id":"aws-k8s-1.17","version_id":"1.0.5","build_id":"ace13fe9","arch":"x86_64"}

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[zmrow]

🍰

[tjkirch]

^ This push rebases on develop.

[tjkirch]

ace13fe

The following push (which GitHub doesn't show distinctly) adds the above commit, which reorganizes the buildsys file to use argument vecs instead of strings, so that we can handle arbitrary characters. This is intended to address @samuelkarp's concern.