Document how to increase root disk size when for example mgmt1 runs out of disk space

[cmd-ntrf]

After experimenting, for OpenStack, the root disk blockstorage can be extended, but should not be extended via Terraform because it would trigger a replacement of the instance (destroy+create). These manual steps were successfully applied twice to extend the root disk of a mgmt1 instance:

1. shutdown all instances
2. extend the instance root disk volume using OpenStack CLI or Horizon
3. reboot all instances.