Upgrade to Apache Karaf 4.2.8 to support Kafka log appender

This is required for CORD. Karaf 4.2.6 explicitelly prevents loading of
some of the bundles required to support shipping logs via Karaf.

Change-Id: I8ae525e6c8a04c0381c5e3bac39de1a7b85b4446
diff --git a/tools/package/onos-prep-karaf b/tools/package/onos-prep-karaf
index c1986b3..315aa13 100755
--- a/tools/package/onos-prep-karaf
+++ b/tools/package/onos-prep-karaf
@@ -12,7 +12,7 @@
 KARAF_PATCHES=$5
 SANDBOX=${6:-.}
 
-KARAF_VERSION="4.2.6"
+KARAF_VERSION="4.2.8"
 ONOS_SECURITY_MODE="false"
 
 PREFIX="onos-$ONOS_VERSION"