Drop fixed_network dependency from CONF

The fixed_network module has very little dependency from config;
removing it so that it may be used by the preprov_creds module.

Change-Id: I24de72a0a997d2824b72338e31fec5329d601074
diff --git a/tempest/test.py b/tempest/test.py
index 7b508ac..a9c8ba7 100644
--- a/tempest/test.py
+++ b/tempest/test.py
@@ -612,8 +612,8 @@
             admin_creds = cred_provider.get_admin_creds()
             admin_manager = clients.Manager(admin_creds)
             networks_client = admin_manager.compute_networks_client
-        return fixed_network.get_tenant_network(cred_provider,
-                                                networks_client)
+        return fixed_network.get_tenant_network(
+            cred_provider, networks_client, CONF.compute.fixed_network_name)
 
     def assertEmpty(self, list, msg=None):
         self.assertTrue(len(list) == 0, msg)