Cluster pcs commands fail with python exception: "ModuleNotFoundError: No module named 'cryptography"

Issue

Many commands ran against the cluster using the pcs command set, fail and report a python exception: ModuleNotFoundError: No module named 'cryptography'.

The stack observed with the error may change depending on the pcs command ran, but reach fail with the same exception:

$ pcs status --full
-----------------------------------------8<----------------------------------------- 
Traceback (most recent call last):
  File "/usr/sbin/pcs", line 11, in <module>
    load_entry_point('pcs==0.10.15', 'console_scripts', 'pcs')()
  File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 476, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2700, in load_entry_point
    return ep.load()
  File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2318, in load
    return self.resolve()
  File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2324, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/usr/lib/python3.6/site-packages/pcs/entry_points/cli.py", line 9, in <module>
    from pcs.app import main
  File "/usr/lib/python3.6/site-packages/pcs/app.py", line 6, in <module>
    from pcs import (
  File "/usr/lib/python3.6/site-packages/pcs/utils.py", line 38, in <module>
    from pcs.cli.common.lib_wrapper import Library
  File "/usr/lib/python3.6/site-packages/pcs/cli/common/lib_wrapper.py", line 10, in <module>
    from pcs.lib.commands import (
  File "/usr/lib/python3.6/site-packages/pcs/lib/commands/cluster.py", line 16, in <module>
    from pcs.common import (
  File "/usr/lib/python3.6/site-packages/pcs/common/ssl.py", line 5, in <module>
    from cryptography import x509
ModuleNotFoundError: No module named 'cryptography' <---

$ pcs quorum status
-----------------------------------------8<----------------------------------------- 
Traceback (most recent call last):
  File "/usr/sbin/pcs", line 11, in <module>
    load_entry_point('pcs==0.10.15', 'console_scripts', 'pcs')()
  File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 476, in load_entry_point
    return get_distribution(dist).load_entry_point(group, name)
  File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2700, in load_entry_point
    return ep.load()
  File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2318, in load
    return self.resolve()
  File "/usr/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2324, in resolve
    module = __import__(self.module_name, fromlist=['__name__'], level=0)
  File "/usr/lib/python3.6/site-packages/pcs/entry_points/cli.py", line 9, in <module>
    from pcs.app import main
  File "/usr/lib/python3.6/site-packages/pcs/app.py", line 6, in <module>
    from pcs import (
  File "/usr/lib/python3.6/site-packages/pcs/utils.py", line 38, in <module>
    from pcs.cli.common.lib_wrapper import Library
  File "/usr/lib/python3.6/site-packages/pcs/cli/common/lib_wrapper.py", line 10, in <module>
    from pcs.lib.commands import (
  File "/usr/lib/python3.6/site-packages/pcs/lib/commands/cluster.py", line 16, in <module>
    from pcs.common import (
  File "/usr/lib/python3.6/site-packages/pcs/common/ssl.py", line 5, in <module>
    from cryptography import x509
ModuleNotFoundError: No module named 'cryptography' <---