oauthserver: grab unsupported overrides from a prefixed tree

[stlaz]

Also removes duplicate code

/assign @vareti

[stlaz]

/retest

[openshift-ci-robot]

@stlaz: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
ci/prow/e2e	86bdba6	link	/test e2e

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[openshift-bot]

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/lifecycle stale

[stlaz]

/remove-lifecycle stale
this is still relevant

[sttts]

/lgtm

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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