Updated jenkins-slave-zap

[esune]

This PR includes changes to update the jenkins-slave-zap image.

Notable changes:

• Updated to OpenJDK 11
• Updated Webswing version to the latest available
• When building the new image, a new version of python-owasp-zap-v2.4 will be pulled-in (same package name as before)
• Reorganized a bit Dockerfiles
• Updated to the latest version available in the Zap repository all the Zap scripts (see files in zap folder(s)).
• Removed rpm binary files: dependencies are now installed from online repository

Recommendation for release: when building and releasing the new image, it would be a good idea to tag the jenkins-slave-zap images with the corresponding versions of SonarQube (existing version would be 6, the new version would be 7) as it is not guaranteed that using different versions on the SonarQube server and the slave will yield successful results.
I had mixed success when testing, I believe an old version of the slave with a new version of the SonarQube server may work, but it is not true the other way around.
Tagging versions would allow users to pick their combination of server and jenkins-slave images to use.

Tagging @WadeBarnes for review, not sure who else needs to be involved to build and publish a new image.