Make a clear split between Packer and Terraform provisioning steps:
the scripts in the `packer/linux` directory are run when we build the
AMI whereas the stuff in shared are run at Terraform provisioning time.
Merging all runtime provisioning scripts into a single script for each
of server/client solves the following:
* Userdata scripts can't take arguments, they can only be templated
and that means we have to do TF escaping in bash/powershell scripts.
* TF provisioning scripts race with userdata scripts.
