Add initial support for Flatcar Linux

[jkroepke]

Issue #, if available:

Description of changes:

Add support for Flatcar Linux. https://www.flatcar.org/

Since Flatcar is an immutable OS system, I mirror the Bottlerocket logic around the updater plugin.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[Thor-Bjorgvinsson]

Thank you for posting this CR! couple of questions:

1. With this current change, does the agent detect the correct OS Platform? The agents send the detected OS Platform to health and can be seen either in the debug logs or from describe-instance-information API
2. Does Flatcar need to support agent update? Dependency for agent update is that the agent is run as a service via systemd which I believe is not in environments where agent is running as a service, (see here how bottle rocket added a precondition to determine if agent update should be supported)

Once the PR is final, please rebase with mainline and squash commits into one

[jkroepke]

Hi @Thor-Bjorgvinsson

With this current change, does the agent detect the correct OS Platform?

Yes, I also add a test which successfully detect the OS Platform based on the /etc/os-release files.

Dependency for agent update is that the agent is run as a service via systemd which I believe is not in environments where agent is running as a service

On Flatcar, amazon-ssm-agent runs as systemd server and the binaries are mutable. But there is no pkg manager.

Here is the unit file:

[Unit]
Description=amazon-ssm-agent
After=network-online.target

[Service]
Type=simple
WorkingDirectory=/usr/share/oem
ExecStart=/usr/share/oem/bin/amazon-ssm-agent
KillMode=process
Restart=on-failure
RestartForceExitStatus=SIGPIPE
RestartSec=15min

[Install]
WantedBy=multi-user.target

[jkroepke]

@Thor-Bjorgvinsson @aguman-aws Can you please take a look here again?

[jkroepke]

@rhaoran-aws @Thor-Bjorgvinsson @aguman-aws I would appreciate an review here.

[jkroepke]

@cjinaws could you take a look here?

[jkroepke]

@gianniLesl @cjinaws @rhaoran-aws @Thor-Bjorgvinsson @aguman-aws I would appreciate a review here.

[gianniLesl]

Change looks good, will test internally and pull in for release

[jkroepke]

@gianniLesl Do you already have some feedback?

[gianniLesl]

Hi, this is in the pipeline for release later this month

[jkroepke]

@gianniLesl Do you have any information, when the release is availible?

[gianniLesl]

This changed was pushed with v3.2.1542.0