Remove interface json and set skip_test method

According to last comments to previous patch:
https://review.openstack.org/#/c/117458/28
 - remove interface=json
 - add skip checks to separate method

Change-Id: Id596b8d33533088f79e20293a980ad2ec80198a6
diff --git a/tempest/api/network/test_dhcp_ipv6.py b/tempest/api/network/test_dhcp_ipv6.py
index e7ba6bb..5e786a0 100644
--- a/tempest/api/network/test_dhcp_ipv6.py
+++ b/tempest/api/network/test_dhcp_ipv6.py
@@ -25,7 +25,6 @@
 
 
 class NetworksTestDHCPv6(base.BaseNetworkTest):
-    _interface = 'json'
     _ip_version = 6
 
     """ Test DHCPv6 specific features using SLAAC, stateless and
@@ -39,7 +38,7 @@
     """
 
     @classmethod
-    def resource_setup(cls):
+    def skip_checks(cls):
         msg = None
         if not CONF.network_feature_enabled.ipv6:
             msg = "IPv6 is not enabled"
@@ -47,6 +46,9 @@
             msg = "DHCPv6 attributes are not enabled."
         if msg:
             raise cls.skipException(msg)
+
+    @classmethod
+    def resource_setup(cls):
         super(NetworksTestDHCPv6, cls).resource_setup()
         cls.network = cls.create_network()