Initial end-to-end tests for dynamic host volumes. This includes tests for two
workflows:
* One where a dynamic host volume is created by a plugin and then mounted by a job.
* Another where a dynamic host volume is created out-of-band and registered by a
job, then mounted by another job.
This changeset also moves the existing `volumes` E2E test package to the
better-named `volume_mounts`.
Ref: https://hashicorp.atlassian.net/browse/NET-11551
