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

install/ui/host.js

@@ -146,6 +146,7 @@ IPA.host.entity = function(spec) {
                     fields: [
                         {
                             name: 'ip_address',
+                            validators: [ IPA.ip_address_validator() ],
                             metadata: IPA.get_command_option('host_add', 'ip_address')
                         },
                         {