Run index task in ldap updater only when needed
    
    When LDAP updater detected an update instruction in indexing tree, it run
    an indexing task and waited until it ends. However, the task was run
    regardless of the update instruction result. This lead to unnecessary
    index tasks being defined and waited for which makes the whole LDAP
    last longer.
    
    Execute indexing task only when an index add/update instruction is
    successful.
    
    https://fedorahosted.org/freeipa/ticket/2866