Allow "--override-nexthop" and "tunnel setup cross subnets" to be enabled together

[yydzhou]

Is your feature request related to a problem? Please describe.

In current kube-router implementation, enabling of "override-nexthop" will prevent the tunnel from being setup for cross subnet case, regardless of whether the tunnel is enabled or not. In our case, we have multiple group of nodes, and each group of nodes has their own subnets. At same time, each node has multiple upstream routers to peer to support both public and private networking. So both of the override nexthop and tunnel cross different subnet need to work at same time.

Here is the code reference
https://github.com/cloudnativelabs/kube-router/blob/master/pkg/controllers/routing/network_routes_controller.go#L562

Describe the solution you'd like
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The two feature should be controlled independently by user with the provided parameters.

Describe alternatives you've considered
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Additional context
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https://kubernetes.slack.com/archives/C8DCQGTSB/p1610724771001400?thread_ts=1609875847.077500&cid=C8DCQGTSB

[aauren]

This is related to #749

[yydzhou]

The required change is merged already. So closing this ticket.

[aauren]

Thanks!