Fail early if people attempt to add zuul vars or secrets

We can block this in config loading before jobs start. Leave the other
validation as well to prevent jobs from passing variables as part of the
return process.

Change-Id: I071a1fcd6037ab0dca78d83ff69b77907d0ccae6
diff --git a/zuul/configloader.py b/zuul/configloader.py
index c925024..f175bab 100644
--- a/zuul/configloader.py
+++ b/zuul/configloader.py
@@ -460,6 +460,8 @@
             else:
                 secret_name = secret_config['name']
                 secret = layout.secrets[secret_config['secret']]
+            if secret_name == 'zuul':
+                raise Exception("Secrets named 'zuul' are not allowed.")
             if secret.source_context != job.source_context:
                 raise Exception(
                     "Unable to use secret %s.  Secrets must be "
@@ -574,6 +576,8 @@
 
         variables = conf.get('vars', None)
         if variables:
+            if 'zuul' in variables:
+                raise Exception("Variables named 'zuul' are not allowed.")
             job.updateVariables(variables)
 
         allowed_projects = conf.get('allowed-projects', None)
diff --git a/zuul/executor/server.py b/zuul/executor/server.py
index 8f4cc1d..cb139c6 100644
--- a/zuul/executor/server.py
+++ b/zuul/executor/server.py
@@ -1275,6 +1275,8 @@
         secrets = playbook['secrets']
         if secrets:
             if 'zuul' in secrets:
+                # We block this in configloader, but block it here too to make
+                # sure that a job doesn't pass secrets named zuul.
                 raise Exception("Defining secrets named 'zuul' is not allowed")
             jobdir_playbook.secrets_content = yaml.safe_dump(
                 secrets, default_flow_style=False)
@@ -1379,6 +1381,8 @@
         # TODO(mordred) Hack to work around running things with python3
         all_vars['ansible_python_interpreter'] = '/usr/bin/python2'
         if 'zuul' in all_vars:
+            # We block this in configloader, but block it here too to make
+            # sure that a job doesn't pass variables named zuul.
             raise Exception("Defining vars named 'zuul' is not allowed")
         all_vars['zuul'] = args['zuul'].copy()
         all_vars['zuul']['executor'] = dict(