Cleanup Derby properties in the generated project POM

In ARCHE-141, the following properties were introduced in the generated project POM:

<derby.system.home>${project.basedir}/target/derby</derby.system.home>
<derby.stream.error.file>${project.basedir}/target/derby/derby.log</derby.stream.error.file>

Actually, 'derby.system.home' is not used because it is overriden by 'repo.path'. So this property could be removed.
Another little improvement could be to put the derby.log file in the 'repo.path' folder (any other location under the target folder is also fine though).
So at the end the 2 properties above could be replaced by only this one:

<derby.stream.error.file>${repo.path}/derby.log</derby.stream.error.file>