[ami]: reduce snapshot size to minimum allowed

Every AMI we create today hold 30Gb EBS snapshot, if we take into
account the fact that we copy those images to other regions and have
multiple images (dev, debug, releases) it is adding up to our cost.

This commit reduce the size of the snapshot to 8Gb (it's the minimum
value allowed for a snapshot)

In addtion, it's reducing also the time for build process from 30
minutes to ~12 minutes (which is also good)

Refs: scylladb/scylla-pkg#3712

packer/scylla.json

@@ -4,40 +4,6 @@
       "name": "aws",
       "type": "amazon-ebs",
       "access_key": "{{user `access_key`}}",
-      "ami_block_device_mappings": [
-        {
-          "device_name": "/dev/sdb",
-          "virtual_name": "ephemeral0"
-        },
-        {
-          "device_name": "/dev/sdc",
-          "virtual_name": "ephemeral1"
-        },
-        {
-          "device_name": "/dev/sdd",
-          "virtual_name": "ephemeral2"
-        },
-        {
-          "device_name": "/dev/sde",
-          "virtual_name": "ephemeral3"
-        },
-        {
-          "device_name": "/dev/sdf",
-          "virtual_name": "ephemeral4"
-        },
-        {
-          "device_name": "/dev/sdg",
-          "virtual_name": "ephemeral5"
-        },
-        {
-          "device_name": "/dev/sdh",
-          "virtual_name": "ephemeral6"
-        },
-        {
-          "device_name": "/dev/sdi",
-          "virtual_name": "ephemeral7"
-        }
-      ],
       "ami_name": "{{user `image_name`| clean_resource_name}}",
       "associate_public_ip_address": "{{user `associate_public_ip_address`}}",
       "sriov_support": true,
@@ -47,7 +13,7 @@
         {
           "delete_on_termination": true,
           "device_name": "/dev/sda1",
-          "volume_size": 30
+          "volume_size": 8
         }
       ],
       "region": "{{user `region`}}",