Certificates relation fails importing dns

designate certificates relation fails importing python3-dnspython

Charm helpers imports dnspython lazily https://github.com/juju/charm-helpers/blob/master/charmhelpers/contrib/network/ip.py#L512 . If import fails it uses apt to install the python3-dnspython package but designate is set to use a venv and not use system packages.

2019-08-27 04:45:05 INFO juju-log certificates:105: Invoking reactive handler: reactive/layer_openstack_api.py:67:default_setup_certificates
2019-08-27 04:45:06 INFO juju-log certificates:105: Installing python3-dnspython with options: ['--option=Dpkg::Options::=--force-confold']
2019-08-27 04:45:06 DEBUG certificates-relation-joined Reading package lists...
2019-08-27 04:45:06 DEBUG certificates-relation-joined Building dependency tree...
2019-08-27 04:45:06 DEBUG certificates-relation-joined Reading state information...
2019-08-27 04:45:06 DEBUG certificates-relation-joined python3-dnspython is already the newest version (1.15.0-1).
2019-08-27 04:45:06 DEBUG certificates-relation-joined The following package was automatically installed and is no longer required:
2019-08-27 04:45:06 DEBUG certificates-relation-joined grub-pc-bin
2019-08-27 04:45:06 DEBUG certificates-relation-joined Use 'apt autoremove' to remove it.
2019-08-27 04:45:06 DEBUG certificates-relation-joined 0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
2019-08-27 04:45:07 ERROR juju-log certificates:105: Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-designate-1/.venv/lib/python3.6/site-packages/charmhelpers/contrib/network/ip.py", line 513, in get_hostname
    import dns.reversename
ModuleNotFoundError: No module named 'dns'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-designate-1/.venv/lib/python3.6/site-packages/charms/reactive/__init__.py", line 73, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-designate-1/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-designate-1/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-designate-1/.venv/lib/python3.6/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-designate-1/charm/reactive/layer_openstack_api.py", line 73, in default_setup_certificates
    for cn, req in instance.get_certificate_requests().items():
  File "/var/lib/juju/agents/unit-designate-1/.venv/lib/python3.6/site-packages/charms_openstack/charm/classes.py", line 560, in get_certificate_requests
    json_encode=False).get('cert_requests', {})
  File "/var/lib/juju/agents/unit-designate-1/.venv/lib/python3.6/site-packages/charmhelpers/contrib/openstack/cert_utils.py", line 121, in get_certificate_request
    req.add_hostname_cn()
  File "/var/lib/juju/agents/unit-designate-1/.venv/lib/python3.6/site-packages/charmhelpers/contrib/openstack/cert_utils.py", line 86, in add_hostname_cn
    'cn': get_hostname(ip),
  File "/var/lib/juju/agents/unit-designate-1/.venv/lib/python3.6/site-packages/charmhelpers/contrib/network/ip.py", line 519, in get_hostname
    import dns.reversename
ModuleNotFoundError: No module named 'dns'