Add method to read changes from LDIF
    
    modifications_from_ldif will read LDIF file and changes in LDIF will
    be cached until parse() is called. After calling parse() method changes
    will be applied into destination LDIF.
    
    Only changetype modify is supported, the default operation is add.
    
    https://fedorahosted.org/freeipa/ticket/4949
    
    Also fixes:
    https://fedorahosted.org/freeipa/ticket/4048
    https://fedorahosted.org/freeipa/ticket/1930
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>