Create default network for more compute tests

Chipping away at the referenced bug, this commit uses the
create_default_network class-level variable introduced via [1] to
trigger setup_credentials to create a default network for another test
case in which the problem was observed.

[1] Ia34fe3b41261de02918713c2a948e6c56554bf6f
Partial-Bug: #1844568

Change-Id: Ic500aff5adda66328c292c936eaa097253360ea4
diff --git a/tempest/api/compute/servers/test_servers_negative.py b/tempest/api/compute/servers/test_servers_negative.py
index f42bb9c..7fa30b0 100644
--- a/tempest/api/compute/servers/test_servers_negative.py
+++ b/tempest/api/compute/servers/test_servers_negative.py
@@ -555,6 +555,7 @@
 
 
 class ServersNegativeTestMultiTenantJSON(base.BaseV2ComputeTest):
+    create_default_network = True
 
     credentials = ['primary', 'alt']