Make cache compatible with encrypted settings

This saves the id value of the setting into the cache
if the setting is encrypted. That can then be combined
with the secret_key in order to decrypt the setting,
without having to make an additional query to the database.

awx/conf/models.py

@@ -74,6 +74,10 @@ class Setting(CreatedModifiedModel):
     def get_cache_key(self, key):
         return key
 
+    @classmethod
+    def get_cache_id_key(self, key):
+        return '{}_ID'.format(key)
+
 
 import awx.conf.signals  # noqa