* 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
