DHCP6 Tests fail to remove DVR ports

The method _clean_network() only removes centralized
router interface ports since it uses only 'network:router_interface'
as a valid router interface port.

Change-Id: Ic647d27c5c4e3bcb8ec0349e31684204e3d0c569
Closes-bug: #1403983
diff --git a/tempest/api/network/test_dhcp_ipv6.py b/tempest/api/network/test_dhcp_ipv6.py
index 5bc63d0..7c7371d 100644
--- a/tempest/api/network/test_dhcp_ipv6.py
+++ b/tempest/api/network/test_dhcp_ipv6.py
@@ -60,7 +60,7 @@
         resp, body = self.client.list_ports()
         ports = body['ports']
         for port in ports:
-            if (port['device_owner'] == 'network:router_interface'
+            if (port['device_owner'].startswith('network:router_interface')
                 and port['device_id'] in [r['id'] for r in self.routers]):
                 self.client.remove_router_interface_with_port_id(
                     port['device_id'], port['id']