Set max_template_size to heat's default value

Align default with heat, as this is what most people will be testing.
Patch to make devstack not override the default value:
I77549f2b5e953ff712c50a2b372f6b04725d5eb0

Change-Id: I77c5b59ae9c78489ebd08fd54c865118d3ec00ed
diff --git a/tempest/config.py b/tempest/config.py
index db923e9..062eb12 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -410,7 +410,7 @@
                default=None,
                help="Name of existing keypair to launch servers with."),
     cfg.IntOpt('max_template_size',
-               default=32768,
+               default=524288,
                help="Value must match heat configuration of the same name."),
 ]