Use outputDirectory from AbstractMavenReport

A reporting plugin shall generate reports always in the reporting output
dir. Inherit from parent and don't set manually otherwise you end up in
issues like this:
Only in it/analyze-testDependencyWithNonTestScope/target: css
Only in it/analyze-testDependencyWithNonTestScope/target:
dependency-analysis.html
Only in it/analyze-testDependencyWithNonTestScope/target: images

which is non-sense since this is clearly site content even if rendered
in standalone mode.

The output dir will be created by the caller/system.