Why does Leapp upgrade fails with 'NameError: global name 'dnf' is not defined' ?

Issue

• Leapp preupgrade command fails with below traceback.

Process Process-242:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python2.7/site-packages/leapp/repository/actor_definition.py", line 72, in _do_run
    actor_instance.run(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/leapp/actors/__init__.py", line 289, in run
    self.process(*args)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/actors/scansubscriptionmanagerinfo/actor.py", line 21, in process
    scanrhsm.scan()
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/rhsm.py", line 103, in wrapper
    return f(*args, **kwargs)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/actors/scansubscriptionmanagerinfo/libraries/scanrhsm.py", line 9, in scan
    info = rhsm.scan_rhsm_info(context)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/rhsm.py", line 103, in wrapper
    return f(*args, **kwargs)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/rhsm.py", line 382, in scan_rhsm_info
    info.available_repos = get_available_repo_ids(context)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/rhsm.py", line 171, in get_available_repo_ids
    repofiles = repofileutils.get_parsed_repofiles(context)
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/repofileutils.py", line 74, in get_parsed_repofiles
    cmd = ['find', '-L'] + get_repodirs() + ['-maxdepth', '1', '-type', 'f', '-name', '*.repo']
  File "/usr/share/leapp-repository/repositories/system_upgrade/common/libraries/repofileutils.py", line 54, in get_repodirs
    with dnf.base.Base() as base:
NameError: global name 'dnf' is not defined