[CI][Nightly] Support local debugging for multi-node CI test cases (#4489)

### What this PR does / why we need it?
 This patch mainly doing the following things:
1. Make k8s/lws optional for multi-node testing, allowing developers to
run multi-node tests locally by actively passing in the IP addresses of
all nodes.
2. Allows passing a custom proxy script path in the config file to load
the proxy.

- vLLM version: v0.11.2

---------

Signed-off-by: wangli <wangli858794774@gmail.com>

tests/e2e/nightly/multi_node/config/models/DeepSeek-V3.yaml

@@ -20,6 +20,9 @@ env_common:
   HCCL_BUFFSIZE: 1024
   SERVER_PORT: 8080
   NUMEXPR_MAX_THREADS: 128
+  DISAGGREGATED_PREFILL_PROXY_SCRIPT: "examples/disaggregated_prefill_v1/load_balance_proxy_server_example.py"
+# For None kubernetes deployment, list the IPs of all nodes used in order as follow
+# cluster_hosts: []
 disaggregated_prefill:
   enabled: true
   prefiller_host_index: [0]