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Add a flag to customize config relative dir

So while migrating nodee2e configs to test-infra, I found out that I'd need to have a better support for [user-data](https://github.com/kubernetes/test-infra/blob/master/jobs/e2e_node/image-config.yaml#L11). However it's not wise to use an [absolute path](https://github.com/kubernetes/test-infra/blob/master/jobs/config.json#L9309), having the config dir to be configurable will be a better solution here, and as well for later on support run local node tests from test-infra.

Currently the job references to the image configs from test-infra, but read metadata from kubernetes, which is wrong :-\


/assign @yguo0905 @Random-Liu