Fix print_hostnames of inventory.py (#8554)

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.