[Jenkins-infra] disk space paging is not actionable

[Kohsuke Kawaguchi]

I just got paged that the disk space is below 1GB, but neither the alert or
the monitor view on Datadog shows which host & volume the alert is firing
for. This is not actionable.

Looking at the configuration, {{host.name}} is meant to expand to the host
name in question, but clearly not such expansion is happening.
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