Ds provision

@pentaho-nbaker, please review this PR

The first commit fixes a raising NPE and is pretty simple.

The second is shipped with the re-implementation of AclNodeHelper. Now, it does not call runAsSystem(). Instead it utilises a two-level hierarchy. The top of it is an ACL Node, that is always visible for Authenticated users. The second level, I have called it ACL store, just stores the ACL.

I also updated the interface's javadocs pointing out that most of the methods should be called with repository administrator role. And last but not least - I've updated the corresponding test