Fix print_hostnames of inventory.py

When trying to run print_hostnames of inventory.py, it outputs the following
error:

 $ CONFIG_FILE=./test-hosts.yaml python3 ./inventory.py print_hostnames
 Traceback (most recent call last):
   File "./inventory.py", line 472, in <module>
     sys.exit(main())
   File "./inventory.py", line 467, in main
     KubesprayInventory(argv, CONFIG_FILE)
   File "./inventory.py", line 92, in __init__
     self.parse_command(changed_hosts[0], changed_hosts[1:])
   File "./inventory.py", line 415, in parse_command
     self.print_hostnames()
   File "./inventory.py", line 455, in print_hostnames
     print(' '.join(self.yaml_config['all']['hosts'].keys()))
 KeyError: 'all'

because it is missed to load a hosts config file before printing hostnames.
This fixes the issue.

contrib/inventory_builder/inventory.py

@@ -88,6 +88,8 @@ def __init__(self, changed_hosts=None, config_file=None):
             if changed_hosts[0] == "add":
                 loadPreviousConfig = True
                 changed_hosts = changed_hosts[1:]
+            elif changed_hosts[0] == "print_hostnames":
+                loadPreviousConfig = True
             else:
                 self.parse_command(changed_hosts[0], changed_hosts[1:])
                 sys.exit(0)
@@ -105,6 +107,10 @@ def __init__(self, changed_hosts=None, config_file=None):
                 print(e)
                 sys.exit(1)
 
+        if changed_hosts[0] == "print_hostnames":
+            self.print_hostnames()
+            sys.exit(0)
+
         self.ensure_required_groups(ROLES)
 
         if changed_hosts: