Incorporate recent Alpine edge changes

• standardize on 'linux-virt' kernel
  • no longer need 'kernel_flavor' variable
  • always install 'aws-ena-driver' package
  • always enable ena_support
• switch to 'variables.yaml' for config
  • update build/convert script to stringify certain keys that may contain arrays
  • copy from 'variables.yaml-default' if it doesn't exist
• drop 'vpc' variable, using 'subnet' derives the proper VPC to use
• fix chrony.conf (all pool.ntp.org references are changed to 169.254.169.123)
• update README.md caveats