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/common/preprov_creds.py b/tempest/common/preprov_creds.py
index 5b81148..74cc3f0 100644
--- a/tempest/common/preprov_creds.py
+++ b/tempest/common/preprov_creds.py
@@ -339,7 +339,7 @@
         try:
             network = fixed_network.get_network_from_name(
                 net_name, compute_network_client)
-        except exceptions.InvalidConfiguration:
+        except exceptions.InvalidTestResource:
             network = {}
         net_creds.set_resources(network=network)
         return net_creds