Update GKE documentation to 1.12

[roberthbailey]

Part of #717.

Notes:

1. As in Update minikube documentation and dev tooling to 1.12 #895, the role binding is no longer required with Kubernetes 1.12+ so I've removed the part.
2. I've removed the flag to disable basic auth now that it is just setting the default behavior.

[agones-bot]

Build Succeeded 👏

Build Id: 960eba4d-ae5a-4e6f-b59d-04fab13ff589

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

• image: gcr.io/agones-images/agones-controller:0.12.0-2584d7d
• image: gcr.io/agones-images/agones-sdk:0.12.0-2584d7d
• image: gcr.io/agones-images/agones-ping:0.12.0-2584d7d
• Linux C++ SDK (build): agonessdk-0.12.0-2584d7d-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-0.12.0-2584d7d.zip

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

• https://2584d7d-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/897/head:pr_897 && git checkout pr_897
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=0.12.0-2584d7d

[markmandel]

Not sure if for this PR, but I assume this should also mean it should also be removed from here?
https://github.com/googleforgames/agones/blob/master/build/includes/google-cloud.mk#L68

[markmandel]

LGTM!

[markmandel]

LGTM!

[agones-bot]

Build Failed 😱

Build Id: c1d18c01-7dac-43e9-b97c-bfe4dcf637a4

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[roberthbailey]

Not sure if for this PR, but I assume this should also mean it should also be removed from here?
https://github.com/googleforgames/agones/blob/master/build/includes/google-cloud.mk#L68

Might as well remove it since we are switching to 1.12. If we don't do it now, it'll probably be forgotten. I've removed it and pushed an updated branch.

[agones-bot]

Build Failed 😱

Build Id: 533b2048-4d89-4263-b74b-2b8275965e00

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[agones-bot]

Build Failed 😱

Build Id: 5f25e0bf-bc20-4ed4-a86f-1dd117a508cb

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[agones-bot]

Build Failed 😱

Build Id: 925c8b65-6c12-4239-97d4-07f9fd35f8b4

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[agones-bot]

Build Failed 😱

Build Id: 7fe63332-901d-4a79-b8f7-5929ff0f174d

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[agones-bot]

Build Failed 😱

Build Id: 34045ab1-f867-4bcf-ba49-4a79fd334591

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[agones-bot]

Build Succeeded 👏

Build Id: 6b61679b-a509-49f9-8976-f179504ba47f

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

• image: gcr.io/agones-images/agones-controller:0.12.0-166ede5
• image: gcr.io/agones-images/agones-sdk:0.12.0-166ede5
• image: gcr.io/agones-images/agones-ping:0.12.0-166ede5
• Linux C++ SDK (build): agonessdk-0.12.0-166ede5-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-0.12.0-166ede5.zip

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

• https://166ede5-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/897/head:pr_897 && git checkout pr_897
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=0.12.0-166ede5