This fixes few cases where driver eventor goroutines are leaked during
normal operations, but especially so in tests.
This change makes few modifications:
First, it switches drivers to use `Context`s to manage shutdown events.
Previously, it relied on callers invoking `.Shutdown()` function that is
specific to internal drivers only and require casting. Using `Contexts`
provide a consistent idiomatic way to manage lifecycle for both internal
and external drivers.
Also, I discovered few places where we don't clean up a temporary driver
instance in the plugin catalog code, where we dispense a driver to
inspect and validate the schema config without properly cleaning it up.
The plugin loader supplies a version map to ensure the Nomad agent
can support the plugins at the version they specify. The map was
incorrectly mapping the driver type to the device API supported
version identifier. This currently does not cause a bug as both
device and driver versions are the same string. This could cause
problems in the future, however, if either plugin interface were
to change and require version updates.
