Added IP address validator to Host and DNS record adder dialog
    
    Also fixed minor issues reagarding IP addresses or multivalued field:
     - removed unnecessary method overrides from multivalued_field
     - fixed extract_child_value method in multivalued_widget to return '' instead of empty arrays when value array is empty
     - net.js - changed method name and error message from 'trailing zeros' to 'leading zeros'
    
    https://fedorahosted.org/freeipa/ticket/1466