Install the service defaults file on Debian

[jcharaoui]

Short description

Tweak the packaging so that the defaults file is actually installed on Debian.

The systemd unit file definition already refers to /etc/default/puppet.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)