support needed from md-sal infra to safely remove common portion of instanceId when all leaf users are removed

In the process of moving from deprecated DataBrokerService to the new DataBroker, many implementations began using the 'createParents=true' param; to
not having to rely on a certain prefix of the tree to being present.

That behavior is handy but may now be generating a potential leak (aka obsolete reference) when all users of the 'shared' part of the tree are done using it.

For the implementations, it would not be proper to remove from the 'root' as it could potentially delete other configuration data that is hanging off the same tree; so the best it can do is to remove it's 'authoritative' portion.

Ref link: https://gist.github.com/864043e0a96c4e704497