[integ-test] Modify test to test cluster without Internet connection using GPU instance

[hanwen-cluster]

Description of changes

This test covers the cookbook fix: aws/aws-parallelcluster-cookbook#3086

This commit also removes a comment that is no longer true

Tests

The following test has passed:

test-suites:
  networking:
    test_cluster_networking.py::test_cluster_in_no_internet_subnet:
      dimensions:
        - regions: ["use1-az2"]
          instances: ["g5.xlarge"]
          oss: ["rhel9"]
          schedulers: ["slurm"]

Checklist

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