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
Merge "Set max_template_size to heat's default value"