Make karaf-empty a POM package

Make karaf-empty a POM package

opendaylight-karaf-empty is currently a (default) JAR package, which
causes maven-install-plugin to try to install a JAR at all costs. It
ends up copying pom.xml to opendaylight-karaf-empty-....jar which
breaks anything then processing the JARs and expecting them to be
valid (e.g. for signing). Declaring that opendaylight-karaf-empty is a
POM package still produces the Karaf distributions along with the POM,
but avoids generating an invalid JAR.

Change-Id: I766acd4fd12002a05968b286a5290461c409c92e
Signed-off-by: Stephen Kitt <skitt@redhat.com>