Add a docker-compose for just the stores (#13)

* - Added docker-compose-storage.yaml, just the infrastructure bits
- Tidied storage invocation

* Util, sits on chunker output and reports histogram of chunk sizes

templates/components/stores/cassandra.jsonnet

@@ -0,0 +1,20 @@
+local base = import "../base.jsonnet";
+local images = import "../images.jsonnet";
+{
+    volumes +: {
+	cassandra: {},
+    },
+    services +: {
+
+	cassandra: base + {
+	    image: images.cassandra,
+	    ports: [
+		"9042:9042"
+	    ],
+	    volumes: [
+		"cassandra:/var/lib/cassandra"
+	    ],
+	},
+
+    },
+}