Add a settings.xml for use with the BuildHive job

We have a BuildHive job to provide continuous integration and testing here:

	https://buildhive.cloudbees.com/job/maven-nar/job/maven-nar-plugin/

Unfortunately, the cpptasks-parallel artifact is not deployed anywhere,
and therefore the BuildHive job only succeeds sporadically (whenever the
cpptasks-parallel job has been built successfully, for a limited amount
of time).

BuildHive allows specifying a settings.xml file for such cases. Since we
deployed cpptasks-parallel to the ImageJ Maven repository, let's have
that file so that our BuildHive job fails only when the code has a
problem.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>