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.
A failing script in a `remote-exec` provisioner's `inline` stanza
won't fail the provisioning step. This lets us continue on to execute
tests against potentially broken deployments, rather than letting us
know the provisioning itself failed.
When multiple developers are working on e2e testing, it helps to be
able to identify which infrastructure belongs to which Nomad SHA and
which developer. This adds tags to the EC2 instances.
