Alpine v3.8, etc.

• switch to alpine v3.8
• add public_ip variable, setting to 'true' allows packer to build from outside AWS
• use smallest instance_type (t2.nano) and volume_size (1 GiB)
• eu-west-3 region is live; ap-northeast-3 requires subscription
• no longer need setup_staging_repos function...
  • tiny-ec2-bootstrap is available in main since v3.8
  • aws-ena-driver-vanilla is only available in edge/testing
• switched to linux-vanilla since linux-hardened is no longer available and linux-virt does not have NVME available
  • TODO? make kernel choice selectable (significant memory/disk savings if linux-virt can be used)