fix: facts being gathered unnecessarily

Cause: The comparison of the present facts with the required facts is
being done on unsorted lists.

Consequence: The comparison may fail if the only difference is the
order.  Facts are gathered unnecessarily.

Fix: Use `difference` which works no matter what the order is.  Ensure
that the fact gathering subsets used are the absolute minimum required.

Result: The role gathers only the facts it requires, and does
not unnecessarily gather facts.

Signed-off-by: Rich Megginson <rmeggins@redhat.com>

defaults/main.yml

@@ -132,3 +132,9 @@ __network_required_facts:
   - distribution_version
   - os_family
   - python
+
+# the subsets of ansible_facts that need to be gathered in case any of the
+# facts in required_facts is missing; see the documentation of
+# the 'gather_subset' parameter of the 'setup' module
+__network_required_facts_subsets: "{{ ['!all', '!min'] +
+  __network_required_facts }}"