Use jetty 9.4 property configuration mechanism

In response to discussions on #77,  the next RC of jetty-9.4 will contain a feature described here https://github.com/eclipse/jetty.project/issues/1139

This will allow jetty configuration to be externalized from the pom.xml and to be contained in a single file, including the properties that need to be edited for #77 and #82.   