Allow setting "UseExistingCluster" via OS Env

[ichekrygin]

Currently, controller-runtime does not support setting UseExistingCluster dynamically via environment variables (similarly to envKubeAPIServerBin or envEtcdBin, etc.).

It would be very convenient and helpful to provide a uniform way in which users can dynamically set the option to UseExistingCluster via the environment.

[DirectXMan12]

/kind feature

[DirectXMan12]

/priority important-longterm

[DirectXMan12]

/good-first-issue

[k8s-ci-robot]

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[DirectXMan12]

done