intento to backport OAK-4055 to branch 1.2

[Tommaso Teofili]

Hi all,

I'd like to backport OAK-4055 [1] to branch 1.2 to make it possible to use
a Solr index for a subtree instead of for the whole repository.

Regards,
Tommaso

[1] : http://issues.apache.org/jira/browse/OAK-4055