[Test] test_multi_az_fsx: use flexible instance types to prevent the risk of ICEs and instance types not being supported.

[gmarciani]

Description of changes

In test_multi_az_fsx: use flexible instance types to prevent the risk of ICEs and instance types not being supported.

Tests

SUCCESS

test-suites:
  storage:
    test_fsx_lustre.py::test_multi_az_fsx:
      dimensions:
        - regions: ["eu-west-2"]
          instances: {{ common.INSTANCES_DEFAULT_X86 }}
          oss: [{{ LUSTRE_OS_X86_1 }}]
          schedulers: ["slurm"]

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