JCR-3748 - Allow configuring S3Backend programatically

As CachingDataStore invokes init directly its not possible to use the other init method. Added another way where clients can provide the required properties while creating S3DataStore

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/trunk@1577481 13f79535-47bb-0310-9956-ffa450edef68