Ensure reasonable defaults for config files for instance system service

[pun-ky]

https://github.com/Cognifide/gradle-aem-plugin/blob/master/src/asset/localInstance/defaults/service/systemd.conf

by default is oneshot, but forking seems to be better, however there are more constraints like https://askubuntu.com/questions/1044464/new-main-pid-does-not-belong-to-service-and-pid-file-is-not-owned-by-root
when User= is set (when AEM is running as root, the issue is gone)

https://github.com/Cognifide/gradle-aem-plugin/blob/master/src/asset/localInstance/defaults/service/sysvinit.sh
these tpl to be tested