Garbage collect old generated MachineConfigs

[jlebon]

Right now, previously generated MachineConfigs just hang around forever. We need to figure out how to clean them safely. There was related code in the render controller (see #318) that deleted the ownerRef of the pool on other MachineConfigs, but it didn't actually work (and anyway removing an ownerReference in itself doesn't trigger Kubernetes to delete the MC). The tricky part here is making sure we don't delete MCs that are still used by any MCD instance (where "used" means as a current, pending, or desired config).

Related: #301
Related: #318
Related: #347

[ashcrow]

/kind enhancement

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/lifecycle stale

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/lifecycle rotten
/remove-lifecycle stale

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[openshift-ci-robot]

@openshift-bot: Closing this issue.

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/close

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