QA spec for Auto DevOps should use 10GB GKE disk

What does this MR do?

This was defaulting to 100GB according to https://cloud.google.com/sdk/gcloud/reference/container/clusters/create#--disk-size which is really not ideal since we haven't automated clean up per gitlab-qa#298 (closed)

Also 10GB is the minimum from what I can tell with the CLI (since I tried 8GB and it failed):

gcloud --project gitlab-paas container clusters create --machine-type n1-highmem-2 --zone us-central1-a dgriffith-2019-07-30t15h09m02s --disk-size 8GB --enable-basic-auth
ERROR: (gcloud.container.clusters.create) argument --disk-size: value must be greater than or equal to 10GB; received: 8GB

Does this MR meet the acceptance criteria?

Conformity

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Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

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