Merge changes from topic "rhel79-upgrade"

* changes:
  [DOC] Update Rhel/Centos version in docs to 7.9
  [ANSIBLE] Disable DNS management in Network Manager only if it's installed
  [CICDANSIBLE] Update heat template "image_name" parameter description
  [MOLECULE] Upgrade roles tests image to Centos7.9
  [MOLECULE] Upgrade playbook tests image to Centos7.9
  [BUILD] Run Centos7.9 docker image for rpm download