Use network from get_tenant_network in scenario

In the scenario tests create_server helper method the returned network
from get_tenant_network() was not being used it was returned and the
method to add it to the kwargs dict was called, but the resulting
updated dict was never used. This patch corrects the oversight by
updating the kwargs dict to ensure we actually pass the right network
into the create server call.

Change-Id: I1976ed6eb2edfa77971bd908fe47514c1a801b98
diff --git a/tempest/scenario/manager.py b/tempest/scenario/manager.py
index ce640b1..eb2276a 100644
--- a/tempest/scenario/manager.py
+++ b/tempest/scenario/manager.py
@@ -186,7 +186,8 @@
         if create_kwargs is None:
             create_kwargs = {}
         network = self.get_tenant_network()
-        fixed_network.set_networks_kwarg(network, create_kwargs)
+        create_kwargs = fixed_network.set_networks_kwarg(network,
+                                                         create_kwargs)
 
         LOG.debug("Creating a server (name: %s, image: %s, flavor: %s)",
                   name, image, flavor)