Better logging on explorer modules

Logging usage is weird and does not ease up debugging as logger classes are not specific to the class using the log.

For instance the following log:

ERROR [BaseNuxeoArtifactDocAdapter] Unable to find NXBundleGroup for id grp:org.nuxeo.ecm.platform

is actually coming from class RepositoryDistributionSnapshot: BaseNuxeoArtifactDocAdapter is actually using a protected log, shared by all subclasses.

==> clean this all up and use new log4j logger everywhere.