make sure ARCHIVA_BASE is not overridden

For some reason MAPPASM-175 changed it from set.default.X to set.X, which
means it can't be changed.

git-svn-id: https://svn.apache.org/repos/asf/archiva/branches/2.0.0-RC@1562696 13f79535-47bb-0310-9956-ffa450edef68