Service unit file should be automatically installed

[razvanphp]

There should be no need to run graylog-collector-sidecar -service install as this can be done by deb when detecting a systemd system. This is what most packages already do, including graylog-server.

As a side-bug from installing this programatically with ansible, dir /etc/graylog/collector-sidecar/generated/ should not be part of the deb, since having etckeeper git repo would cause uncommited changes, I already moved the generated folder to /var/lib and it seems it automatically creates the missing directories, so the old one can just be removed from the deb.

Looking forward for this deb package being included in the repo. Thank you!