service files belong in /lib/systemd/system not /etc/systemd/system

[jsoref]

Kibana version:
6.5.4

Server OS version:
debian stretch 9.6

Original install method (e.g. download page, yum, from source, etc.):
apt http://artifacts.elastic.co/packages/6.x/apt

Describe the bug:
https://unix.stackexchange.com/questions/206315/whats-the-difference-between-usr-lib-systemd-system-and-etc-systemd-system

Steps to reproduce:

1. install kibana (or the others), or look at the packaging: https://github.com/elastic/kibana/blob/94d99d63f3157d4a08eea83aa2489158b66a5e87/src/dev/build/tasks/os_packages/service_templates/systemd/etc/systemd/system/kibana.service

Expected behavior:
service files should live in /lib/systemd/system
Files that go in /etc/systemd/system should be placed by an admin manually (e.g. to override a package or because a package doesn't provide one).

I may provide a PR for this.

I hit it while trying to decide which files I needed to manually migrate when migrating an install

[elasticmachine]

Pinging @elastic/kibana-operations