Fix distribution fact on DragonFly (#17619)

* By default, ansible_distribution is not set on DragonFly systems,
  preventing some distribution-specific tests from being written

* This commit fixes the issue by returning the quite logical value
  of "DragonFly" when appropriate

lib/ansible/module_utils/facts.py

@@ -673,7 +673,7 @@ class Distribution(object):
         self.facts['distribution'] = self.system
         self.facts['distribution_release'] = platform.release()
         self.facts['distribution_version'] = platform.version()
-        systems_implemented = ('AIX', 'HP-UX', 'Darwin', 'FreeBSD', 'OpenBSD', 'SunOS')
+        systems_implemented = ('AIX', 'HP-UX', 'Darwin', 'FreeBSD', 'OpenBSD', 'SunOS', 'DragonFly')
 
         self.facts['distribution'] = self.system
 
@@ -777,6 +777,9 @@ class Distribution(object):
         else:
             self.facts['distribution_version'] = 'release'
 
+    def get_distribution_DragonFly(self):
+        pass
+
     def get_distribution_Slackware(self, name, data, path):
         if 'Slackware' not in data:
             return False  # TODO: remove