Change Environment to EnvironmentFile in systemd unit file

[A1EF]

This PR relative to #367. Currently if you need to change environment variables you have to edit systemd unit file. This is quite inconvenient to do. It would be nice to have separate config file with reference in systemd unit file.

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
✅ A1EF
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}