Fail build if classpath contains duplicates

Fail build if classpath contains duplicates

duplicate-finder-maven-plugin fails build if duplicate classpath entries
see https://github.com/basepom/duplicate-finder-maven-plugin/

This is currently not enforced, but projects opt-in to this quality via:

  <properties><duplicate-finder.skip>false

FTR: There are a few other tools in this space, incl. e.g.
http://www.mojohaus.org/extra-enforcer-rules/banDuplicateClasses.html
and http://tattletale.jboss.org/ and http://jhades.github.io, but
duplicate-finder-maven-plugin fits best here.

See https://git.opendaylight.org/gerrit/#/c/50851/ for a related util.

Bug: 7662
Change-Id: I79142549ba7c739ce39285d35f179172d7b099a6
Signed-off-by: Michael Vorburger <vorburger@redhat.com>