tripleo-ansible-inventory fails with ModuleNotFoundError: No module named 'decorator'

Issue

• Updating the ansible inventory on director is failing with the following:

$ tripleo-ansible-inventory
Traceback (most recent call last):
  File "/usr/bin/tripleo-ansible-inventory", line 26, in <module>
    import os_client_config
  File "/usr/lib/python3.6/site-packages/os_client_config/__init__.py", line 19, in <module>
    from os_client_config import cloud_config
  File "/usr/lib/python3.6/site-packages/os_client_config/cloud_config.py", line 17, in <module>
    from openstack import _log
  File "/usr/lib/python3.6/site-packages/openstack/__init__.py", line 17, in <module>
    import openstack.connection
  File "/usr/lib/python3.6/site-packages/openstack/connection.py", line 188, in <module>
    from openstack.cloud import openstackcloud as _cloud
  File "/usr/lib/python3.6/site-packages/openstack/cloud/openstackcloud.py", line 21, in <module>
    import dogpile.cache
  File "/usr/lib/python3.6/site-packages/dogpile/cache/__init__.py", line 1, in <module>
    from .region import CacheRegion  # noqa
  File "/usr/lib/python3.6/site-packages/dogpile/cache/region.py", line 22, in <module>
    from decorator import decorate
ModuleNotFoundError: No module named 'decorator'