[ClusterClass Operations] Cover rebase in E2E tests

[salasberryfin]

This is issue is part of the epic #387.

Description

With the addition of the rebase controller (#638) based on the ClusterUpgradeGroup resource, it needs to be validated as part of the E2E. Ideally, we should validate that:

• No cluster can be used as ClusterTarget in two different ClusterUpgradeGroup.
• One/multiple Cluster created from ClusterClass can be updated via rebase. This can simply be having two slightly different ClusterClass and checking that Cluster can be successfully updated to use a different one than the one it was originally created from.