system: deny reading other user's authorised keys

I believe that users should not be able to peek at each other
authentication details (authorised keys and date of last change).

Change-Id: I6c158f0727e3f8edcdbd1028066bd09ecb31bced
diff --git a/yang/czechlight-authentication.json b/yang/czechlight-authentication.json
index d291970..6604b16 100644
--- a/yang/czechlight-authentication.json
+++ b/yang/czechlight-authentication.json
@@ -8,8 +8,8 @@
         ],
         "rule": [
           {
-            "name": "Allow executing actions of current user",
-            "access-operations": "exec",
+            "name": "Allow reading and executing actions in the context of the current user",
+            "access-operations": "exec read",
             "module-name": "czechlight-system",
             "action": "permit",
             "path": "/czechlight-system:authentication/users[name=$USER]"