Set externalTrafficPolicy as Local for agones-allocator

[osterante]

Is your feature request related to a problem? Please describe.
When using two node pools in an Agones cluster, one for Agones and one for GameServers, allocation requests sometimes fail during the process of reducing nodes in the GameServers’ node pool, especially when reducing many nodes. I think it shouldn't be affected by changes in the GameServers’ node pool.

Describe the solution you'd like
Set the externalTrafficPolicy for the agones-allocator service to Local from Cluster(default).
https://cloud.google.com/kubernetes-engine/docs/concepts/service-load-balancer?hl=ja#health_check

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

Additional context
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