Make `node-preload-images` `pollingTimeout` configurable

[xdu31]

What would you like to be added:

Right now pollingTimeout in node-preload-images is hardcoded with 15 mins timeout,

perf-tests/clusterloader2/pkg/imagepreload/imagepreload.go

Line 47 in eba398a

pollingTimeout = 15 * time.Minute

.

Suggest to make another flag as node-preload-images-timeout to make it configurable

Why is this needed:

On a large cluster, e.g. with 5k nodes, this can time out

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