Make unstable-neutron job more stable

We are trying to use to much volume space. As we are using random()
it's different every time.

So reduce just arguments

Change-Id: I3599e608d1a24872e9930fcb61891b6f5e94100b

rally-jobs/unstable-neutron.yaml

@@ -94,7 +94,7 @@
           volume_type: false
           size:
               min: 1
-              max: 2
+              max: 1
       runner:
         type: "constant"
         times: 3
@@ -117,7 +117,7 @@
           volume_type: true
           size:
               min: 1
-              max: 2
+              max: 1
       runner:
         type: "constant"
         times: 3
@@ -142,7 +142,7 @@
       args:
           size:
               min: 1
-              max: 2
+              max: 1
           nested_level:
               min: 2
               max: 2