QA spec for Auto DevOps should 10GB GKE disk

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
https://gitlab.com/gitlab-org/gitlab-qa/issues/298

qa/qa/service/kubernetes_cluster.rb

@@ -29,6 +29,7 @@ module QA
           #{auth_options}
           --enable-basic-auth
           --region #{Runtime::Env.gcloud_region}
+          --disk-size 10GB
           && gcloud container clusters
           get-credentials
           --region #{Runtime::Env.gcloud_region}