Fix project_network_v6_mask_bits value in case of Contrail env

For contrail env "project_network_v6_cidr = 2003::/120", so we
need to change the "project_network_v6_mask_bits" value from
default '64' up to '124'.

Change-Id: I7a359bdc5a0e50d63982147b5f80dfce4fec729f
Closes-bug: PROD-25897
diff --git a/_modules/runtest/tempest_sections/network.py b/_modules/runtest/tempest_sections/network.py
index b9d8cf9..fe06b0d 100644
--- a/_modules/runtest/tempest_sections/network.py
+++ b/_modules/runtest/tempest_sections/network.py
@@ -91,7 +91,12 @@
 
     @property
     def project_network_v6_mask_bits(self):
-        pass
+        contrail_enabled = conditions.BaseRule('*.opencontrail.control.enabled',
+                                               'eq', True, multiple='any')
+        if contrail_enabled.check(self.pillar):
+            return '124'
+        else:
+            pass
 
     @property
     def project_networks_reachable(self):