Bug 6523: Use opendaylight-karaf-empty for SingleFeatureTest

Bug 6523: Use opendaylight-karaf-empty for SingleFeatureTest

This makes feature-parent to use the same karaf container
as integration/distribution uses for testing its features.

The downside is developers experiencing
daily download of a big snapshot artifact.

A solution which replaces org.apache.aries.proxy.impl
in maven-surefire-plugin default (non-snapshot) karaf container would be preferable.

Change-Id: I6cd4f0ee4c9883e0376f262bd2f09d1bc1c29911
Signed-off-by: Vratko Polak <vrpolak@cisco.com>