Using gp3 instead of gp2 for Ubuntu

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: Idb7e30b86197ad5effbee1bd3508fc18c7a13e79
diff --git a/packer_ubuntu.json b/packer_ubuntu.json
index 535935c..01f6e93 100644
--- a/packer_ubuntu.json
+++ b/packer_ubuntu.json
@@ -32,8 +32,8 @@
       "launch_block_device_mappings": [
         {
           "device_name": "/dev/sda1",
-          "volume_type": "gp2",
-          "volume_size": "50",
+          "volume_type": "gp3",
+          "volume_size": "30",
           "delete_on_termination": true
         }
       ]