Lower concurrency on rally port job

Previous runs are showing that creating ports under this high load
ends up taking >5 secs per port on average. Lets set to 4, which is
still double the api_worker count in some cases for the current gate.

Change-Id: I05a0d28f5b035684e07288825f5b704a843dc9d7

rally-jobs/neutron-neutron.yaml

@@ -73,7 +73,7 @@
       runner:
         type: "constant"
         times: 40
-        concurrency: 20
+        concurrency: 4
       context:
         users:
           tenants: 1