Define worker root device size.

util/packer/jenkins_worker.json

@@ -25,7 +25,13 @@
     "security_group_id": "{{user `security_group`}}",
     "tags": {
       "delete_or_keep": "{{user `delete_or_keep`}}"
-    }
+    },
+    "launch_block_device_mappings": [{
+      "delete_on_termination": true,
+      "device_name": "/dev/sda1",
+      "volume_size": "40",
+      "volume_type": "gp2"
+    }]
   }],
   "provisioners": [{
     "type": "shell",