ephemeral disk: migrate should imply sticky

[tgross]

The ephemeral_disk block's migrate field allows for best-effort migration of the ephemeral disk data to new nodes. The documentation says the migrate field is only respected if sticky=true, but in fact if client ACLs are not set the data is migrated even if sticky=false.

The existing behavior when client ACLs are disabled has existed since the early implementation, so "fixing" that case now would silently break backwards compatibility. Additionally, having migrate not imply sticky seems nonsensical: it suggests that if we place on a new node we migrate the data but if we place on the same node, we throw the data away!

Update so that migrate=true implies sticky=true as follows:

• The failure mode when client ACLs are enabled comes from the server not passing along a migration token. Update the server so that the server provides a migration token whenever migrate=true and not just when sticky=true too.
• Update the scheduler so that migrate implies sticky.
• Update the client so that we check for migrate || sticky where appropriate.
• Refactor the E2E tests to move them off the old framework and make the intention of the test more clear.

---

Fixes #12314

E2E test results:

$ go test -v -count=1 ./nodedrain
=== RUN   TestNodeDrain
=== RUN   TestNodeDrain/IgnoreSystem
    node_drain_test.go:239: alloc has drained from node=b39d3578 after 17.453100094s
=== RUN   TestNodeDrain/EphemeralMigrate
    node_drain_test.go:239: alloc has drained from node=b39d3578 after 16.54164811s
=== RUN   TestNodeDrain/KeepIneligible
--- PASS: TestNodeDrain (50.41s)
    --- PASS: TestNodeDrain/IgnoreSystem (25.99s)
    --- PASS: TestNodeDrain/EphemeralMigrate (22.94s)
    --- PASS: TestNodeDrain/KeepIneligible (1.26s)
PASS
ok      github.com/hashicorp/nomad/e2e/nodedrain        50.413s

[schmichael]

Looks great!