fixes #8261 - use Default Organization for certificate org

If the user's not explicitly setting it, then they don't care whatsoever about the field. "Default Organization," "krsrwasdfjsadf", or "SomeOrg" all sound like just fine default values to me.

The only benefit to the way I've done it here is `capsule-certs-generate` is consistent, and shows (default: "Default Organization").   Under your proposal it will say `(default: "Katello")` and offer to the user `subscription-manager register --organization="Katello"`, which is fixable I guess --  we just don't offer the user anything except an example instead of something useful to cut and paste. 

I can't put `$::foreman::initial_organization` as the value in `answers.capsule-certs-generate.yaml` , can I?