Please review (3rd) ticket 47823: RFE enforce attributes uniqueness over several subtrees

[thierry bordaz]

Ticket is https://fedorahosted.org/389/ticket/47823

Fix is 
https://fedorahosted.org/389/attachment/ticket/47823/0003-Ticket-47823-attri...
Taking into account Rich reviews that simplify the handling of plugins 
config, that is now in the pblock SLAPI_PLUGIN_PRIVATE.
Thank you very much for these tips.

Test case is 
https://fedorahosted.org/389/attachment/ticket/47823/0002-Ticket-47823-test-...