Add systemd value for agent.base_image setting (system tests configuration)

[mrodm]

What does this PR do?

Add a new option systemd to the agent.base_image

Why is it important?

This new setting is intended to run the system tests using an Elastic Agent image without the synthetics capabilities. It would run with an Elastic Agent image "just" with all the binaries for running Beats.

elastic-package implementation https://github.com/elastic/elastic-package/pull/2066

Examples:

Elastic Agent images used while testing with two different values of agent.base_image:

• Setting agent.base_image: systemd:

  $ docker ps --format "{{.Names}} {{.Image}}" |grep elastic-agent
  elastic-package-agent-nginx-access-34488-elastic-agent-1 docker.elastic.co/elastic-agent/elastic-agent:8.15.0-SNAPSHOT
  elastic-package-stack-elastic-agent-1 docker.elastic.co/elastic-agent/elastic-agent-complete:8.15.0-SNAPSHOT
  elastic-package-stack-fleet-server-1 docker.elastic.co/elastic-agent/elastic-agent-complete:8.15.0-SNAPSHOT

• Setting agent.base_image: complete:

  $ docker ps --format "{{.Names}} {{.Image}}" |grep elastic-agent
  elastic-package-agent-nginx-access-46446-elastic-agent-1 docker.elastic.co/elastic-agent/elastic-agent-complete:8.15.0-SNAPSHOT
  elastic-package-stack-elastic-agent-1 docker.elastic.co/elastic-agent/elastic-agent-complete:8.15.0-SNAPSHOT
  elastic-package-stack-fleet-server-1 docker.elastic.co/elastic-agent/elastic-agent-complete:8.15.0-SNAPSHOT

Checklist

• [x] I have added test packages to test/packages that prove my change is effective.
• [x] I have added an entry in spec/changelog.yml.

Related issues

• Relates https://github.com/elastic/elastic-package/pull/2066

[mrodm]

buildkite test this

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: 2ed5a59d870c8f7fb04accea83bc4878883227b9

History

• :broken_heart: Build #775 failed 8ce6080d341b26a5aa62dbdd2f326b8aef014668
• :yellow_heart: Build #774 was flaky 8ce6080d341b26a5aa62dbdd2f326b8aef014668

cc @mrodm