bug 1110343 Fix missing config.network.username

This change switches from using config.network.username to
config.identity.username. config.network.username is no longer available
it appears.

Change-Id: I6b4e445e321635a61d0e47901de9d42efb6cfe35
diff --git a/tempest/tests/network/test_network_basic_ops.py b/tempest/tests/network/test_network_basic_ops.py
index bdebced..d297c98 100644
--- a/tempest/tests/network/test_network_basic_ops.py
+++ b/tempest/tests/network/test_network_basic_ops.py
@@ -186,9 +186,9 @@
         cls.check_preconditions()
         cfg = cls.config.network
         cls.tenant_id = cls.manager._get_identity_client(
-            cfg.username,
-            cfg.password,
-            cfg.tenant_name).tenant_id
+            cls.config.identity.username,
+            cls.config.identity.password,
+            cls.config.identity.tenant_name).tenant_id
         # TODO(mnewby) Consider looking up entities as needed instead
         # of storing them as collections on the class.
         cls.keypairs = {}