Improve dnsrecord interactive help
    
    Add 2 new features to DNS record interactive help to increase its
    usability and also make its behavior more consistent with standard
    parameter interactive help:
    
    1) Ask for missing DNS parts
    When a required part of a newly added DNS record was missing, we
    just returned a ValidationError. Now, the interactive help rather
    asks for all missing required parts of all DNS records that were
    being added by its parts.
    
    2) Let user amend invalid part
    When an interactive help asked for a DNS record part value and
    user enters an invalid value, the entire interactive help exits
    with an error. This may upset a user if he already entered several
    correct DNS record part values. Now, the help rather tells user
    what's wrong and give him an opportunity to amend the value.
    
    https://fedorahosted.org/freeipa/ticket/2386