Activating Maven reporting tools

This PR is the achievement of what I began and presented during the code-sprint week at Bolsena, the main idea is to make use of Maven tools to generate reports about the code (quality, errors by static analysis, coverage, tests results ...).

This needs some modifications into the pom.xml, so that every generated sites from the subprojects will deploy at the same place (i.e. in the `target/site` from the main project directory. I already run the current modifications onto my own jenkins, if you would like to have a look, the generated content is here:

https://ssl.beneth.fr/jenkins/job/core-geonetwork/site/

Since every poms have been affected, but I needed to modify the `geoserver` submodule, I need to open another PR related to the core-geoserver project.

To generate these reports, you have to launch the following target:

```
$ mvn site-deploy
```

Feedback welcome !