Add a blank space between "eui_ip" and "when".

Change-Id: Ie63120968d554f43a55b617e14b72e2f96b49793
diff --git a/tempest/api/network/test_dhcp_ipv6.py b/tempest/api/network/test_dhcp_ipv6.py
index 0730d58..399954c 100644
--- a/tempest/api/network/test_dhcp_ipv6.py
+++ b/tempest/api/network/test_dhcp_ipv6.py
@@ -135,7 +135,7 @@
             real_ip, eui_ip = self._get_ips_from_subnet(**kwargs)
             self._clean_network()
             self.assertEqual(eui_ip, real_ip,
-                             ('Real port IP %s shall be equal to EUI-64 %s'
+                             ('Real port IP %s shall be equal to EUI-64 %s '
                               'when ipv6_ra_mode=%s,ipv6_address_mode=%s') % (
                                  real_ip, eui_ip,
                                  ra_mode if ra_mode else "Off",