The `nomad_sha`, `nomad_version`, and `nomad_local_binary` variables for the
Nomad provisioning module assumed that only one would be set. By having the
override each other with an explicit precedence, it makes it easier to avoid
problems with Terraform's implicit variables behavior.
Set the expected default values in the `terraform.full.tfvars` to avoid
shadowing by any future changes to the `terraform.tfvars` file.
Update the Makefile to put the `-var` and `-var-file` in the correct order.
We intend to expand the nightly E2E test to cover multiple distros and
platforms. Change the naming structure for "Linux client" to the more precise
"Ubuntu Bionic", and "Windows" to "Windows 2016" to make it easier to add new
targets without additional refactoring.
Most of the time that a human is running the TF provisioning, they want the
"dev cluster" which is going to deploy an OSS sha, with fewer targets and
configuration alternatives. But the default `terraform.tfvars` is the nightly
E2E run. Because the nightly run is automated, there's no reason we can't have
it pick a non-default `terraform.full.tfvars` file and have the default be the
dev cluster.
