Problem Statement

A new startup shell script has been introduced in https://github.com/jenkinsci/docker/pull/762.
The differences are not very big, so we should make this a variable or something.

Acceptance criteria

• only one jenkins.sh file (and no jenkins-jdk11.sh script anymore)
• and specifics for JDK11 should be passed as external variable, so we have only one startup script to maintain.