redesign the hypershift/openshift support

[ffromani]

introducing support for hypershift meant that we now have a wide array of platform checks all around the codebase.
The main difference between the platforms is how we compute NodeGroups. We need to clean up this and possibly
factorize the platform differences in a new pkg/nodegroups package.

[Tal-or]

/cc @Tal-or currently working on the issue