Don't allow auth definition in job to be a list

In experimenting with secrets and reading the configloader it appears
like the auth option is supposed to be a list. However when auth is
provided as a list later handling fails and an 'Unknown configuration
error' is thrown.

I think the default here should be that auth is a dictionary if it is
provided.

Change-Id: I573ad0d819905c0351e7d50bcc3c7b9c4d03e2d1
Signed-off-by: Jamie Lennox <jamielennox@gmail.com>
diff --git a/zuul/configloader.py b/zuul/configloader.py
index a8d643b..fd56fab 100644
--- a/zuul/configloader.py
+++ b/zuul/configloader.py
@@ -227,7 +227,7 @@
                'tags': to_list(str),
                'branches': to_list(str),
                'files': to_list(str),
-               'auth': to_list(auth),
+               'auth': auth,
                'irrelevant-files': to_list(str),
                'nodes': vs.Any([node], str),
                'timeout': int,