After submitting an update, the test ought to wait until the new
allocations are placed. Previously, we'd use the original to-be-stopped
allocations and the test fails when attempting to exec.
The E2E framework wraps testify's `require` so that by default we can stop
tests on errors, but the cleanup functions should use `assert` so that we
continue to try to cleanup the test environment even if there's a failure.
Exercises host volume and Docker volume functionality for the `exec` and `docker`
task driver, particularly around mounting locations within the container and
how this can be used with `template`.
Mahmood Ali