Allow env var overrides for e2e tests

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking

/kind bug

/kind cleanup
/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it:

The ordering of env var parsing meant you couldn't override test values such as image with an environment variable, even though the intent of the code seems to indicate that is the desired result.

This change allows the use of environment variables can now be used!

Which issue(s) this PR fixes:

None

Special notes for your reviewer:
None

[agones-bot]

Build Failed 😱

Build Id: ae8021ad-7d0e-4bd9-8240-7ba4e316725b

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[markmandel]

Doh, broke a thing:

Starting e2e integration test!
go test -mod=vendor -race -parallel=64 agones.dev/agones/test/e2e --kubeconfig /root/.kube/config \
	--gameserver-image=gcr.io/agones-images/udp-server:0.20 \
	--pullsecret= \
	--feature-gates="PlayerTracking=true&ContainerPortAllocation=true"
unknown flag: --kubeconfig
Usage of /tmp/go-build754818963/b001/e2e.test:
      --feature-gates string   Flag to pass in the url query list of feature flags to enable or disable
unknown flag: --kubeconfig
FAIL	agones.dev/agones/test/e2e	0.153s

I'll come back to this later.

[agones-bot]

Build Failed 😱

Build Id: 99d7d42b-4804-41fe-aad4-8e8d1c5f52d8

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[markmandel]

--- FAIL: TestFleetAggregatedPlayerStatus (103.13s)
    fleet_test.go:1270: Could not message GameServer: read udp 192.168.10.2:41816->35.247.4.172:7473: i/o timeout

Looks like it might be a bit of a flake. Might want to put a retry on that operation.

[agones-bot]

Build Succeeded 👏

Build Id: 67710271-2678-44ee-ae8a-d4d75df5e9fa

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

• image: gcr.io/agones-images/agones-controller:1.6.0-9a8ea49
• image: gcr.io/agones-images/agones-sdk:1.6.0-9a8ea49
• image: gcr.io/agones-images/agones-ping:1.6.0-9a8ea49
• Linux C++ SDK (build): agonessdk-1.6.0-9a8ea49-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.6.0-9a8ea49.zip

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

• https://9a8ea49-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/GoogleCloudPlatform/agones.git pull/1566/head:pr_1566 && git checkout pr_1566
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.6.0-9a8ea49

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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• OWNERS [markmandel,roberthbailey]

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