command-line specification of plugin parameters doesn't work when plugin is configured in POM

For instance, when the modello plugin has:

<configuration>
<model>maven.mdo</model>
<version>4.0.0</version>
</configuration>

using -Dmodel=maven-other.mdo -Dversion=5.0.0 will not override the configuration in the pom.xml.

Command-line specifications should always override other configuration, where overrides can be made deterministically. I'm not sure that we can be deterministic here, if multiple <goal/> level configs are made (for modello:java, f.e.).

We need to fix this if it is reasonable to do so, or else document the hell out of it if not.