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.

contrib/inventory_builder/inventory.py

@@ -41,6 +41,7 @@ from ruamel.yaml import YAML
 
 import os
 import re
+import subprocess
 import sys
 
 ROLES = ['all', 'kube-master', 'kube-node', 'etcd', 'k8s-cluster',
@@ -69,6 +70,7 @@ MASSIVE_SCALE_THRESHOLD = int(os.environ.get("SCALE_THRESHOLD", 200))
 
 DEBUG = get_var_as_bool("DEBUG", True)
 HOST_PREFIX = os.environ.get("HOST_PREFIX", "node")
+USE_REAL_HOSTNAME = get_var_as_bool("USE_REAL_HOSTNAME", False)
 
 # Configurable as shell vars end
 
@@ -167,6 +169,7 @@ class KubesprayInventory(object):
 
         # FIXME(mattymo): Fix condition where delete then add reuses highest id
         next_host_id = highest_host_id + 1
+        next_host = ""
 
         all_hosts = existing_hosts.copy()
         for host in changed_hosts:
@@ -191,6 +194,12 @@ class KubesprayInventory(object):
                     self.debug("Skipping existing host {0}.".format(ip))
                     continue
 
+                if USE_REAL_HOSTNAME:
+                    cmd = ("ssh -oStrictHostKeyChecking=no "
+                           + access_ip + " 'hostname -s'")
+                    next_host = subprocess.check_output(cmd, shell=True)
+                    next_host = next_host.strip().decode('ascii')
+                else:
                     next_host = "{0}{1}".format(HOST_PREFIX, next_host_id)
                     next_host_id += 1
                 all_hosts[next_host] = {'ansible_host': access_ip,