Tweaks on Nexus Unzip plugin (enable on Snapshots)

In order to fully leverage the Nexus Unzip plugin in our mechanism, it would be nice to:

1. Enable this Nexus plugin on snapshots repo. I tried to find an artifact which is a snapshot in the "unzip" folder but didn't find anything
2. Set up the SNAPSHOT alias to have stable URLs: http://wiki.eclipse.org/Tycho/Nexus_Unzip_Plugin#Enabling_virtual_versions_SNAPSHOT_and_RELEASE

As an example, here is an artifact we'd like to consume unzipped with a stable URL: https://repository.jboss.org/nexus/content/repositories/snapshots/org/jboss/tools/locus/update.site/1.0.0-SNAPSHOT/