Use mand SCS flavors in scs cluster-class template.

[garloff]

• Use SCS-2V-4-20s (note the s) as example and default for control plane nodes. We have created the small SSD flavors for exactly this use case, so let's use them. Also note that SCS-2V-4-20 is no longer guaranteed to exist on every SCS (IaaS-compatible-v3+) cloud.
• Likewise shift to SCS-2V-4 (diskless) as default for the worker nodes. This flavor is guaranteed to exist. It will require a root volume, suggest 25GiB.
• Clarify comments on when to use root volumes ...

What this PR does / why we need it:

The current template defaults to flavors that are no longer mandatory. So one more thing to adjust and thus worse user experience.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

• Suggesting SCS-2V-4-20s for controllers (with local SSD) and SCS-2V-4 (diskless!) for workers. Those two are mandatory. Changing the default root volume to 25GiB (slightly larger) to have a bit more headroom (and to easily recognize that the volume got used instead of a local disk)
• I have NOT tested whether the support for diskless flavors works well ... It should.
• Obviously managed KaaS providers would adjust to their cloud, but good defaults are still considered useful, of course

TODOs:

• squash commits
• include documentation
• add unit tests

[janiskemper]

LGTM from my side. @jschoone can review with regards to the SCS context and the flavor naming.