OAK-1502 - Make DataStores available to NodeStores

Changes as per review by Amit Jain. Thanks!

-- Specifying a default DataStore for tests
-- Populating bean properties before init

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1578738 13f79535-47bb-0310-9956-ffa450edef68