Fix Bugs in K8s System Class Introduced During the Adoption of Pydantic

[TaekyungHeo]

Summary

• Pydantic does not support __post_init__. Member variables were not initialized. Replaced __post_init__ with __init__.
• Added monitor_interval.
• Added docstrings for attributes to ensure completeness.
• Fixed a bug when loading Kubernetes configurations.
• Updated unit tests accordingly.

Test Plan

1. CI passes
2. Tested on K8s

$ python cloudaix.py --mode run --system-config ../cloudai/conf/common/system/kubernetes_cluster.toml --test-templates-dir conf/staging/kubernetes/test_template/ --tests-dir conf/staging/kubernetes/test --test-scenario conf/staging/kubernetes/test_scenario/nccl_test.toml
...
[INFO] Logs for pod 'tests-1-launcher-f2wfl' saved to 'results/nccl-test_2024-09-17_14-13-20/Tests.1/0/tests-1-launcher-f2wfl.txt'
[INFO] Logs for pod 'tests-1-worker-0' saved to 'results/nccl-test_2024-09-17_14-13-20/Tests.1/0/tests-1-worker-0.txt'
[INFO] Logs for pod 'tests-1-worker-1' saved to 'results/nccl-test_2024-09-17_14-13-20/Tests.1/0/tests-1-worker-1.txt'
[INFO] All logs concatenated and saved to 'results/nccl-test_2024-09-17_14-13-20/Tests.1/0/stdout.txt'
[INFO] Job completed: Tests.1
[INFO] All test scenario results stored at: results/nccl-test_2024-09-17_14-13-20

[srivatsankrishnan]

LGTM