Support vip announcement filtering in multiple upstream router cases.

[yydzhou]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is and what the feature provides.

Currently, the kube-router supports to peer multiple upstream routers for redundancy purpose as described here (https://github.com/cloudnativelabs/kube-router/blob/master/docs/bgp.md#global-external-bgp-peers). This feature can also be used to support multiple networking case (e.g., cluster has two routers, one for public network and other other for private network).

However, when it is used to handle multiple networking case, there is a limitation. The kube-router will announce all VIPs to all it peered routers. For example, the private vip announcement will be received by the public upstream router and the versus. To better support multiple network case, kube-router should have the capability to filter the VIP announcement so that the public VIP only announced to public router and private VIP only announced to private router.

Describe the solution you'd like A clear and concise description of what you want to happen.

One possible solution is to allow kube-router to config a vip subnet filter to each upstream router. So only VIPs belongs to specified subnet would be announced to the target router.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered.

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