ci: support skip nightly tests by distro/host

[kb-newrelic]

Summary

• Introduce per-distro test-spec.yaml to toggle nightly tests. The idea was that this could be a starting point to also incorporate features like skipping specific metrics and/or listing the metrics to test in the first place.
  • Corresponding nightly test infra is only deployed when enabled, e.g. k8s distro will not need to run on bare EC2s, so the EC2s shouldn't be deployed in the first place.
  • Skipped toggling k8s_node tests for now as both host and k8s will need to support it and when we move to testing k8s in host/gateway mode, we'll need to provide more finegrained control at which point we can incorporate skipping parts of it for the host distro
• Incorporate distro into
  • k8s namespace to minimize interference
  • the hostname to 'namespace' data in NR

Tests

• tf plan looks good

[mailo-nr]

LGTM