Add nodeSelector property to Agones helm chart for Allocator

[yeslayla]

What type of PR is this?
I would think
/kind feature

What this PR does / Why we need it:
When trying to deploy to a cluster with both Linux and Windows containers, Agones needs to be configurable to just select the Linux nodes. The easiest way to do so is with a nodeSelector.

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[agones-bot]

Build Failed 😱

Build Id: 9acc04f5-c607-4110-94f7-4d5b50c8d4e6

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[agones-bot]

Build Failed 😱

Build Id: e042d5f3-8a3e-4fdb-be73-e824c6f35851

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[agones-bot]

Build Succeeded 👏

Build Id: 55abef85-dae5-4e58-bfad-5daa98e1cf5a

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.12.0-88e0238
• image: gcr.io/agones-images/agones-sdk:1.12.0-88e0238
• image: gcr.io/agones-images/agones-ping:1.12.0-88e0238
• Linux C++ SDK (build): agonessdk-1.12.0-88e0238-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.12.0-88e0238.zip

A preview of the website (the last 30 builds are retained):

• https://88e0238-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1946/head:pr_1946 && git checkout pr_1946
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.12.0-88e0238

[markmandel]

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: josephbmanley, markmandel

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Needs approval from an approver in each of these files:

• OWNERS [markmandel]

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[agones-bot]

Build Succeeded 👏

Build Id: b47400f5-285a-4724-9288-71a349d49296

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.12.0-bd3372d
• image: gcr.io/agones-images/agones-sdk:1.12.0-bd3372d
• image: gcr.io/agones-images/agones-ping:1.12.0-bd3372d
• Linux C++ SDK (build): agonessdk-1.12.0-bd3372d-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.12.0-bd3372d.zip

A preview of the website (the last 30 builds are retained):

• https://bd3372d-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1946/head:pr_1946 && git checkout pr_1946
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.12.0-bd3372d