fix k3d-dev.sh SUBNET_ID specifier and public ip

We were doing some testing with k3d-dev.sh and noticed:

1. An old PubSubnet variable was stomping on SUBNET_ID.
2. This didn't work against subnets that do not have "map public ip on launch" enabled.

This MR fixes both issues. Fully tested against our dev account to ensure no new issues were introduced.