Allow to override rancher provider settings

[k0da]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Currently it is only possible to set provider settings over yaml file.

This commit introduces env variables to override URL, token and cluster name.

If particular environment variable is set it overrides value supplied in yaml file.

Which issue(s) this PR fixes

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Allow to override rancher provider setting via environment variables (`RANCHER_TOKEN`, `RANCHER_URL`, `RANCHER_CLUSTER_NAME`)

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

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[k0da]

/retest

[k8s-ci-robot]

@k0da: Cannot trigger testing until a trusted user reviews the PR and leaves an /ok-to-test message.

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/retest

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[mwielgus]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: k0da, mwielgus

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• cluster-autoscaler/OWNERS [mwielgus]

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