Always clean feature pack modules

http://maven.apache.org/plugins/maven-clean-plugin/usage.html#Running_the_Clean_Plugin_automatically_during_a_build

^^^ might be useful for our maven modules that build feature packs.

I often get failed FP builds as i switch from branch to branch and I believe it's because the target dir has stuff left over from a previous build, e.g. layer-spec.xml files etc, that no longer match the current branch. Doing mvn clean install for the whole build is annoying; WF is a huge build. And manually cleaning a few modules is also annoying. But it seems you can turn on a clean as part of the module's own pom.