Create indexes for krbCanonicalName attribute
    
    krbCanonicalName is for a long time among the attributes guarded by uniqueness
    plugins, but there was never an index for it. Now that the attribute is really
    used to store canonical principal names we need to add index for it to avoid
    performance regressions.
    
    https://fedorahosted.org/freeipa/ticket/6100
    
    Reviewed-By: Thierry Bordaz <tbordaz@redhat.com>