node_exporter: Systemd service unit uses incorrect template, leading to ProtectHome bug

[cudevmaxwell]

Issue #13 was never really fixed. I incorrectly assumed something about the behavior of scopes in Jinja2 templates. Thank you @tjdavis3 for uncovering this problem. Fixed in #94, which uses jinja2's namespaces to ensure variable change propagates. I wanted to use something nicer like python's any() or something more clever, but at least it won't be broken.