Add JDK 8 to the base image

This will let users choose the the JDK they want to run. Additionally
JDK 8 is required by LiveOak image.

JDK 7 will remain the default JDK.

Downside: it grows the image by 60MB. Not sure if this matter when we
have a 1GB image though...
Switch jboss/liveoak-server to use the jboss/base image