Run yangvalidator as yang user/group

[SlavomirMazurPantheon]

Problem:
yangvalidator app is currently running as root, which results in permission error in removeUnused.py script while trying to delete old yangvalidator cache files.

Traceback (most recent call last):
  File "/backend/utility/removeUnused.py", line 219, in <module>
    raise e
  File "/backend/utility/removeUnused.py", line 126, in <module>
    shutil.rmtree(os.path.join(yang_validator_cache, dir))
  File "/usr/local/lib/python3.9/shutil.py", line 718, in rmtree
    _rmtree_safe_fd(fd, path, onerror)
  File "/usr/local/lib/python3.9/shutil.py", line 675, in _rmtree_safe_fd
    onerror(os.unlink, fullname, sys.exc_info())
  File "/usr/local/lib/python3.9/shutil.py", line 673, in _rmtree_safe_fd
    os.unlink(entry.name, dir_fd=topfd)
PermissionError: [Errno 13] Permission denied: 'nokia-conf.yang'

These are helper files which are copied into cache directories during validation.

Possible solution:
If application will be running under yang user/group, these files will be also created/copied under this user.