Make run_user for the launch script configurable

[obfischer]

The run user (run_user) used in the launch script should be configurable.

I have some services written with Spring Boot (1.5.6) as an executable jar with an embedded launch script. The jar is owed by the root user (root:root) therefore the service runs as root.

Therefore should the user the application is running under be configurable.

[snicoll]

Duplicate of #10275 - @obfischer no need to create an issue and a PR. A PR is enough, thanks!