Change default root disk size to 100Gi

[DebakelOrakel]

RedHat lowered requirements for OpenShift node root disk size to 100Gi.

Checklist

• Keep pull requests small so they can be easily reviewed.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Link this PR to related issues.

[simu]

Note that this change requires users to ensure that clusters that were setup with terraform-openshift4-exoscale <= v4 to pin the root disk size of their clusters to the old default of 120Gi.

We explicitly don't add disk_size to the ignore_changes list, so that users can resize VM disks via Terraform (e.g. with an approach similar to the approach described in https://kb.vshn.ch/oc4/how-tos/cloudscale/update_compute_flavors.html)

We'll provide detailed upgrade instructions in the openshift4-terraform component.