[OCI] Support reuse existing VCN for SkyServe (#4530) #18926

Job	Run time
python-test (3.8, tests/unit_tests)	3m 17s
python-test (3.8, tests/test_api.py)	36s
python-test (3.8, tests/test_cli.py)	53s
python-test (3.8, tests/test_config.py)	37s
python-test (3.8, tests/test_global_user_state.py)	26s
python-test (3.8, tests/test_jobs.py)	30s
python-test (3.8, tests/test_list_accelerators.py)	46s
python-test (3.8, tests/test_optimizer_dryruns.py)	1m 47s
python-test (3.8, tests/test_optimizer_random_dag.py)	1m 11s
python-test (3.8, tests/test_storage.py)	27s
python-test (3.8, tests/test_wheels.py)	31s
python-test (3.8, tests/test_jobs_and_serve.py)	26s
python-test (3.8, tests/test_yaml_parser.py)	27s
	11m 54s

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