Create default network in volume tests

Volume negative tests create servers without specifying network,
causing the "Multiple possible networks found" exception
when more than one network exists. Enable default network creation
(create_default_network=True) in these tests to avoid this error

Related-prod: PRODX-51554
Change-Id: I2c836d9e96276b084a1af6d323e4a7775cd2ef13
diff --git a/tempest/api/volume/test_volumes_negative.py b/tempest/api/volume/test_volumes_negative.py
index b5b5581..c405a50 100644
--- a/tempest/api/volume/test_volumes_negative.py
+++ b/tempest/api/volume/test_volumes_negative.py
@@ -30,6 +30,8 @@
 class VolumesNegativeTest(base.BaseVolumeTest):
     """Negative tests of volumes"""
 
+    create_default_network = True
+
     @classmethod
     def resource_setup(cls):
         super(VolumesNegativeTest, cls).resource_setup()