Created by: tomalok
- 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