nexus: Move persistent data to make it easier to upgrade

This makes it so that when I upgrade nexus, I don't have to manually copy the persistent data from versions to version. Instead it patches nexus to store that data in the var directory. 

Nexus stores configuration data in a directory called {bundleBaseDir}/sonatype-work/nexus in the default installation (with bundleBaseDir being the home directory for nexus). All configuration changes to nexus are stored in that location. Configuration changes include adding proxy repositories, adding or changing users, etc. Also, all maven artifacts by default are saved in that same directory. By moving this persistent data from the default directory to var/nexus, new versions of nexus will use that data without any manual intervention.