Add USE_REAL_HOSTNAME to inventory.py (#6293)

inventory_builder creates hosts.yaml file with hostnames like "node1",
"node2", etc. Even if specifying override_system_hostname=false, the
output of "kubectl get nodes" shows those hostnames ("node1", etc.)
without using actual hostnames.
To solve this issue, this adds an option USE_REAL_HOSTNAME to get
actual hostnames when creating hosts.yaml file instead of "node1", etc.