Fix rally "volume_type" parameter

Rally test create_snapshot_and_attach_volume
currently fails with:

    "The parameter 'volume_type' is required
     and should not be empty."

Change-Id: Icc5c4538d99e18d210718842db973ca1dfa48b5a

rally-jobs/cinder.yaml

@@ -336,6 +336,7 @@
         size:
           min: 1
           max: 1
+        volume_type: "test"
       runner:
         type: "constant"
         times: 2
@@ -350,6 +351,8 @@
           flavor:
             name: "m1.tiny"
           servers_per_tenant: 1
+        volume_types:
+          - "test"
         api_versions:
           cinder:
             version: 2