redact the license key from settings.LICENSE activity stream entries

see: https://github.com/ansible/tower/issues/2671

awx/conf/models.py

@@ -78,6 +78,14 @@ class Setting(CreatedModifiedModel):
     def get_cache_id_key(self, key):
         return '{}_ID'.format(key)
 
+    def display_value(self):
+        if self.key == 'LICENSE' and 'license_key' in self.value:
+            # don't log the license key in activity stream
+            value = self.value.copy()
+            value['license_key'] = '********'
+            return value
+        return self.value
+
 
 import awx.conf.signals  # noqa