[Test] In the proxy template, allowlist EPEL mirrors dynamically to prevent failures in test_build_image_no_internet.

[gmarciani]

Description of changes

1. In the proxy template used by tests, allowlist EPEL mirrors dynamically to prevent failures in test_build_image_no_internet. This is what a user would do to support the build-image in proxied environments. The proxy configuration is also validated by the proxy client, which verifies that every mirror is allowlisted.
   In this way the build instance can reach whichever EPEL mirror it is assigned.
2. in test_build_image_no_internet , enable Nvidia installation for all OSs, except when DLAMI is used. In this way we can use this test to validate the nvidia installation.

Tests

• SUCCESS manually deployed the proxy stack in personal account and validated that all the mirrors returned by the mirrorlist are now reachable.
• SUCCESS (verified that NVIDIA was enabled on all tests but the one using DLAMI)

test-suites:
  createami:
    test_createami.py::test_build_image_no_internet:
      dimensions:
        # RHEL family (rhel8, rhel9) -> Red Hat Enterprise Linux reservation platform
        - regions: [{{ g4dn_2xlarge_CAPACITY_RESERVATION_2_INSTANCES_3_HOURS_NOPG_rhel9 }}]
          instances: ["g4dn.2xlarge"]
          oss: ["rhel8", "rhel9"]
        # Non-RHEL family (alinux2023, ubuntu2204, ubuntu2404, rocky8, rocky9) -> Linux/UNIX reservation platform
        - regions: [{{ g4dn_2xlarge_CAPACITY_RESERVATION_5_INSTANCES_3_HOURS_NOPG_ubuntu2404 }}]
          instances: ["g4dn.2xlarge"]
          oss: ["alinux2023", "ubuntu2204", "ubuntu2404", "rocky8", "rocky9"]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[gmarciani]

Note for the reviewer.

This is the line that guarantees that NVIDIa is disabled when using DLAMI.