docs: how_to_upgrade_builders.rst update
    
    - suggest using /usr/bin/openstack, so we can set also the
      hw_rng_model image parameter
    - provide alternative link to download ppc64le images
    - remove the playbook update step
      https://infrastructure.fedoraproject.org/cgit/ansible.git/tree/playbooks/hosts/fed-cloud09.cloud.fedoraproject.org.yml
      as that one is not used anymore
    - suggest turning the VM off before creating snapshot