Can you please clarify one thing. You say:
Are you leaving steps out here, or you are indeed installing not from the Elasticsearch provided repository? We do not support the images provided by Ubuntu, and it does not surprise me that they are broken. Instead, you need to use our repositories:
for 6.x: Install Elasticsearch with Debian Package | Elasticsearch Reference [6.0] | Elastic
for 5.x: Install Elasticsearch with Debian Package | Elasticsearch Reference [5.5] | Elastic
for 2.x: Repositories | Elasticsearch Guide [2.4] | Elastic
for 1.x: Repositories | Elasticsearch Guide [1.7] | Elastic
We support these, and they work with systemd.