Migrate to flavor field in spark 1.3.1

Migration to flavor field was done there
I2f57232e9666b2f56ab97595b6c09df69df75c67 but it was not
introduced in Spark 1.3.1 mako template (which was merged before
that commit). Actually that should resolve issues with Spark 1.3.1
jobs on CI.

Change-Id: Ib123df22d7134bfc2e9de99e5531629628f996f7

etc/scenario/sahara-ci/spark-1.3.1.yaml.mako

@@ -4,13 +4,13 @@ clusters:
     image: ${spark_1_3_image}
     node_group_templates:
       - name: master
-        flavor_id: ${ci_flavor_id}
+        flavor: ${ci_flavor_id}
         node_processes:
           - master
           - namenode
         auto_security_group: true
       - name: worker
-        flavor_id: ${ci_flavor_id}
+        flavor: ${ci_flavor_id}
         node_processes:
           - datanode
           - slave