RFR: JDK-8211037: Load jib jars dynamically from JibArtifactResolver

[Erik Joelsson]

We need to avoid always having the jib jars on the classpath when 
running jtreg tests through the makefiles. They interfere with the test 
setup for certain tests. This can be fixed rather easily by dynamically 
loading the classes based on an environment variable set by make.

Bug: https://bugs.openjdk.java.net/browse/JDK-8211037

Webrev: http://cr.openjdk.java.net/~erikj/8211037/webrev.01/index.html

/Erik