SNO: Fix OKD release variable, fix iso url

[Klaas-]

Version(s):

OKD 4.20 / 4.21

Issue:

The OKD sno install did not work with the provided docs, I fixed the commands and added an automatic default of latest stable release

Link to docs preview:

QE review:

• QE has approved this change.

Additional information:

[openshift-ci]

Hi @Klaas-. Thanks for your PR.

I'm waiting for a github.com member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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