Allow changing the reportOutputDirectory from the commandline

We run the javadoc:aggregate task from the CI server to publish javadocs for a certain release. It's not possible now to specifiy the reportOutputDirectory from the commandline by running

mvn -DreportOutputDirectory=/dest/dir javadoc:aggregate

Instead, you have to put this config in a profile/property which exposes too much environment specific info in the pom.

The attached simple patch allows for specifying the reportOutputDirectory on the cmdline while preserving existing behaviour.