Add a build slave that is capable of building Flatpak versions of Eclipse products

I'm filing a Jira ticket as suggested in Mattermost.

In order to explore shipping our Eclipse-based products as Flatpak applications, (initially SCL and maybe eventually also JBDS) I need a build slave for the Jenkins[1] that is capable of generating Flatpaks.

The newly released RHEL 7.6 has a sufficiently new version of flatpak and flatpak-builder packages. Please also ensure there is java-1.8.0-openjdk-devel and a recent maven installed.

Please consider adding such a builder.

[1] https://dev-platform-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/