don't parse subdirs in host/group_vars

... as those are not supported in core vars_plugins/group_vars.py but might be used by other vars_plugins

Without this patch, and having sub directories beneath host/group_vars yields an error, as the core plugin tries to "read" from the directory, and returns a (wrong) error:

```[bash]
[mstr] serge@goldorak:~/acd$ ansible -m ping localhost
localhost | FAILED => file not found: ./hosts/group_vars/all
[mstr] serge@goldorak:~/acd$ ls -ld ./hosts/group_vars/all
drwxrwxr-x 2 serge serge 4096 May 11 00:27 ./hosts/group_vars/all
[mstr] serge@goldorak:~/acd$ ls -ld ./hosts/group_vars/all/all
-rw-rw-r-- 1 serge serge 1386 May 11 00:27 ./hosts/group_vars/all/all
```

(this error returned by utils.parse_yaml_from_file)

In core, it should just ignore subdirectories, as those are not supported (as per previous discussion #2664). Subdirectories should however be allowed to exist, as other vars_plugins might be handling them (as in my usecase using the custom https://github.com/ansible-contrib/ansible-plugins/blob/stable/vars_plugins/group_vars_dirs.py)