Many of our scripts have a non-portable interpreter line for bash and
use bash-specific variables like `BASH_SOURCE`. Update the interpreter
line to be portable between various Linuxes and macOS without
complaint from posix shell users.

Provisions vault with the policies described in the Nomad Vault integration
guide, and drops a configuration file for Nomad vault server configuration
with its token. The vault root token is exposed to the E2E runner so that
tests can write additional policies to vault.

Adds a `nomad_acls` flag to our Terraform stack that bootstraps Nomad ACLs via
a `local-exec` provider. There's no way to set the `NOMAD_TOKEN` in the Nomad
TF provider if we're bootstrapping in the same Terraform stack, so instead of
using `resource.nomad_acl_token`, we also bootstrap a wide-open anonymous
policy. The resulting management token is exported as an environment var with
`$(terraform output environment)` and tests that want stricter ACLs will be
able to write them using that token.

This should also provide a basis to do similar work with Consul ACLs in the
future.