Eirini corrections

[btat]

• There are references to features.suse_default_stack and features.suse_buildpacks which no longer exist in 2.1
  • transitioning from Diego to Eirini requires using install_stacks: [sle]
    • Need to confirm which scenarios need this since the in the 2.1 chart install_stacks: [sle15, cflinuxfs3] already exists
• Eirini will only work on a cluster that has the parameter --cluster-domain set to cluster.local
  • The mention of this without additional context/details is causing confusion (e.g. what does this flag mean/do, is it something set in values.yaml?)
  • We give an brief explanation what this flag does as well as link to the upstream docs on in (as we did in the release notes)
  • Need to add information on which platforms this affects. Some platforms already have this set by default
• We currently ask users to install the Kubernetes Metrics Server due to fix an application metrics reporting issues.
  • Point out CAP has a Stratos Metrics component, which is not the same thing
  • Explain how the Kube Metrics Server interacts with the internal components
  • Explain which "application metrics" are being referred to