Fix YangToSourcesPluginTestIT failure

Fix YangToSourcesPluginTestIT failure

Inspired by
http://blog2.vorburger.ch/2016/05/how-to-make-maven-archetype-plugin.html

The YangToSourcesPluginTestIT obviously does pass on Jenkins, but that
relies on magical environment variables.  I'm sure it also works locally
for those who use the custom ODL settings.xml globally - but I don't; I
just use "mvn -s odl.xml" kind of thing.  This fix makes the
YangToSourcesPluginTestIT pass even then, because it will now re-use the
real "effective" settings XML under which Maven was started.

Change-Id: I1edbed691ac1a01c8bfc1c8178d4836781aba0a1
Signed-off-by: Michael Vorburger <vorburger@redhat.com>