ipapython.sysrestore: Use str methods instead of functions from the string module
    
    For historical reasons, the string module contained some functions
    that mirror methods of the str type. These are eremoved in Python 3.
    
    Use str methods instead.
    
    Part of the work for https://fedorahosted.org/freeipa/ticket/5638
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>