Improve Configurability

• move config variables from alpine-ami.yaml to variables.json-*
  • variables.json-default - ready-for-action original default config
  • variables.json-example - original defaults with comments
• clean up tabs vs. spaces in make_ami.sh
• make_ami.sh handles custom kernel flavor, extra repos, and extra packages
• tweak README.md with regards to aws-ena-driver caveat