increase the timeout for scaling out

[elmiko]

this change makes the timeout 15 minutes for the autoscaler test where we scale out to maximum size. on some platforms, it appears that 5 minutes is not enough time.

[openshift-ci]

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[openshift-ci]

@elmiko: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/security	f46f27d	link	false	/test security

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