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/build/bazel/osgi_features.bzl b/tools/build/bazel/osgi_features.bzl
index 43610f5..90366aa 100644
--- a/tools/build/bazel/osgi_features.bzl
+++ b/tools/build/bazel/osgi_features.bzl
@@ -102,7 +102,7 @@
 <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
 <features xmlns="http://karaf.apache.org/xmlns/features/v1.2.0"
           name="onos-%s">
-    <repository>mvn:org.apache.karaf.features/standard/4.2.6/xml/features</repository>
+    <repository>mvn:org.apache.karaf.features/standard/4.2.8/xml/features</repository>
 ''' % ONOS_VERSION
 
 FEATURES_FOOTER = "</features>"