Allow not specifying routes within virtual service

[dudicoco]

Summary

Currently, if a virtual service has more than one route you must include all of its routes within the rollout spec.
It would be beneficial to have argo rollouts change all the weights of all routes if no routes are specified.

Use Cases

If you have a virtual service with many routes you must keep your rollouts spec in sync with the routes spec, which could be challenging and prone to configuration errors.

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