Modernize Vault integration/e2e test a bit:
- Download from releases.hashicorp.com instead of using a hardcoded list
- Remove old unused make target e2e-test
- Use NOMAD_E2E env var instead of -integration flag
- Add a README
On my machine with ~250 Mbps internet it takes ~400s to download all
Vault binaries.
Michael Schurter