Add method to reserve the external subnet cidrs

Related-Bug: #1903433
Change-Id: Ic2333d1529d7749207bf81fc842963a5e5208306
diff --git a/neutron_tempest_plugin/scenario/test_metadata.py b/neutron_tempest_plugin/scenario/test_metadata.py
index 3897183..c6d7505 100644
--- a/neutron_tempest_plugin/scenario/test_metadata.py
+++ b/neutron_tempest_plugin/scenario/test_metadata.py
@@ -49,6 +49,7 @@
         super(MetadataTest, cls).resource_setup()
         cls.rand_name = data_utils.rand_name(
             cls.__name__.rsplit('.', 1)[-1])
+        cls.reserve_external_subnet_cidrs()
         cls.network = cls.create_network(name=cls.rand_name)
         cls.subnet_v4 = cls.create_subnet(
             network=cls.network, name=cls.rand_name)