[SPARK-3812] [BUILD] Adapt maven build to publish effective pom.

I have tried maven help plugin first but that published all projects in top level pom. So I was left with no choice but to roll my own trivial plugin. This patch basically installs an effective pom after maven install is finished. 

The problem it fixes is described as follows:
If you install using maven 
` mvn install -DskipTests -Dhadoop.version=2.2.0 -Phadoop-2.2 `
Then without this patch the published pom(s) will have hadoop version as 1.0.4. This can be a problem at some point. 
Suggestion: you could publish an empty jar in whatever repo hosts the "org.spark-project" dependencies.