Add a flag to simple-game-server to shutdown after a specified number of seconds

[roberthbailey]

This flag will eventually replace the similar flag that specifies
the time in minutes to give a finer granularity for testing.

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/kind bug
/kind cleanup
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/kind feature

/kind hotfix

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[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cindy52, roberthbailey

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[agones-bot]

Build Succeeded 👏

Build Id: 2d12a54e-3c31-4e59-8349-5cd5c0d60ca1

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

• image: gcr.io/agones-images/agones-controller:1.20.0-d9479c6
• image: gcr.io/agones-images/agones-ping:1.20.0-d9479c6
• Linux C++ SDK (build): agonessdk-1.20.0-d9479c6-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.20.0-d9479c6.zip

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

• https://d9479c6-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/2407/head:pr_2407 && git checkout pr_2407
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.20.0-d9479c6