Reverse check order between WebCert and WebPrincipal in case both are set

In the case that both WebPrincipal and WebCert are set, reverse the
order of checking them so that WebCert is used by default.

www/kojiweb/index.py

@@ -277,10 +277,10 @@ def login(environ, page=None):
     # This matches the original behaviour
     webauth = options['WebAuth']
     if not webauth:
-        if options['WebCert']:
-            webauth = 'ssl'
         if options['WebPrincipal']:
             webauth = 'kerberos'
+        if options['WebCert']:
+            webauth = 'ssl'
 
     if not webauth:
         raise koji.AuthError(