Add ImagePullSecrets for Allocator and Ping (helm)

[aLekSer]

Added ImagePullSecrets, updated docs, tested with private registry.

Closes #1185 .

[agones-bot]

Build Succeeded 👏

Build Id: 38e65395-1d25-4410-bf3f-96a9b35803a8

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

• image: gcr.io/agones-images/agones-controller:1.2.0-41ae631
• image: gcr.io/agones-images/agones-sdk:1.2.0-41ae631
• image: gcr.io/agones-images/agones-ping:1.2.0-41ae631
• Linux C++ SDK (build): agonessdk-1.2.0-41ae631-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.2.0-41ae631.zip

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

• https://41ae631-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1190/head:pr_1190 && git checkout pr_1190
• helm install install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.2.0-41ae631

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aLekSer, pooneh-m

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