Assign residual child node category 'content' to hst:hst/hst:hosts

During the upgrade effort, we learned that OD2 requires additional hst:virtualhostgroup node trees under /hst:hst/hst:hosts. Currently, they would be categorized as config, and therefore, during an upgrade, they would be removed. We should therefore add a residual-child-node-category of 'content' to the hst:hosts node.