The current implementations use of a specific interface to build
FQDN from has the undesired side effect of the ``nova-compute`` and
``neutron-openvswitch`` charms ending up with using different
hostnames in some situations. It may also lead to use of a
identifier that is mutable throughout the lifetime of a deployment.
Use of a specific interface was chosen due to ``socket.getfqdn()``
not giving reliable results (https://bugs.python.org/issue5004).
This patch gets the FQDN by mimicking the behaviour of a call to
``hostname -f`` with fallback to shortname on failure.
Add relevant update from c-h.
Depends-On: I82db81937e5a46dc6bd222b7160ca1fa5b190c10
Change-Id: Ic8f8742261b773484687985aa0a366391cd2737a
Closes-Bug: #1839300
ee709a5ab3