[KARAF-2314] Use a different etc/org.ops4j.pax.url.mvn.cfg file depending if the distribution is a SNAPSHOT or a release

git-svn-id: https://svn.apache.org/repos/asf/karaf/branches/karaf-2.3.x@1483214 13f79535-47bb-0310-9956-ffa450edef68