Improve DNS PTR record validation
    
    Current PTR validation is unclear and may misled the user. This
    patch improves the validation process so that the eventual exception
    is clearer. New check that the PTR record is fully qualified has
    been added to ensure that the reverse zone resolution behaves as
    expected.
    
    Additionally, several strings in the DNS plugin were prepared for
    localization.
    
    https://fedorahosted.org/freeipa/ticket/1129