Prompt for nameserver IP address in dnszone-add

Prompt for nameserver IP address in interactive mode of dnszone-add.

Add a corresponding field to dnszone creation dialog in the web UI.

This parameter is required if and only if:
* New zone is a forward zone
* Nameserver is defined inside the new zone

Add a new unit test to cover this functionality.

https://fedorahosted.org/freeipa/ticket/3603

ipalib/plugins/dns.py

@@ -1781,9 +1781,30 @@ class dnszone_add(LDAPCreate):
         ),
         Str('ip_address?', _validate_ipaddr,
             doc=_('Add forward record for nameserver located in the created zone'),
+            label=_('Nameserver IP address'),
         ),
     )
 
+    def interactive_prompt_callback(self, kw):
+        """
+        Interactive mode should prompt for nameserver IP address only if all
+        of the following conditions are true:
+        * New zone is a forward zone
+        * NS is defined inside the new zone (NS can be given either in the
+          form of absolute or relative name)
+        """
+        if kw.get('ip_address', None):
+            return
+
+        zone = normalize_zone(kw['idnsname'])
+        ns = kw['idnssoamname']
+        relative_ns = not ns.endswith('.')
+        ns_in_zone = self.obj.get_name_in_zone(zone, ns)
+
+        if not zone_is_reverse(zone) and (relative_ns or ns_in_zone):
+            ip_address = self.Backend.textui.prompt(_(u'Nameserver IP address'))
+            kw['ip_address'] = ip_address
+
     def pre_callback(self, ldap, dn, entry_attrs, attrs_list, *keys, **options):
         assert isinstance(dn, DN)
         if not dns_container_exists(self.api.Backend.ldap2):