Remove Jetty 7 and 8 images

[md5]

When I initially created this image, I included Jetty 7 and 8 images because "stable-7" and "stable-8" versions are listed at http://download.eclipse.org/jetty/

However, I later discovered that Jetty 7 and 8 were EOL'd as of the end of 2014. Here's what Webtide's blog post has to say about it:

This year will mark the end of our open source support for Jetty 7 and Jetty 8. Earlier this week we pushed out a maintenance release that only had a handful of issues resolved over the last five months so releases have obviously slowed to a trickle. Barring any significant security related issue it is unlikely we will see more then a release or two remaining on Jetty 7 and Jetty 8. We recommend users update their Jetty versions to Jetty 9 as soon as they are able to work it into their schedule. For most people we work with, the migration has been trivial, certainly nothing on the scale of the migration between foundations.

Removing Jetty 7 and 8 images will have the benefit of allowing the jetty image as a whole to start using a simple JETTY_BASE owned by user jetty by default, thereby fixing #1. Instructions or a variant image can then be provided that starts as root and uses the built-in setuid module to drop privileges to user jetty.

[md5]

I've created the eol/jetty-7 and eol/jetty-8 branches in case any future security updates are needed for the Jetty 7 or 8 images. Those branches may also be helpful for those who need to support older Jetty versions.