Adds Windows targets to the client/allocs metrics tests. Removes the
`allocstats` test, which covers less than these tests and is now
redundant.
Adds a firewall rule to our Windows instances so that the prometheus
server can scrape the Nomad HTTP API for metrics.
Extends the BasicAllocStats test to include a test for Windows
clients, exercising stats via a powershell `raw_exec` job.
Adds `ListLinuxClientNodes` and `ListWindowsClientNodes` utils so that
we can scope tests to run only when Linux or Windows clients are
available. This prevents waiting on timeouts when running a subset of
the tests against a development cluster (vs our nightly test
cluster).