Disallow Heat-only functions in CloudFormation templates

For CloudFormation templates (containing the AWSTemplateFormatVersion key),
allow only the built-in functions defined by CloudFormation. Templates
wishing to use Heat extensions should use the HeatTemplateFormatVersion
key instead.

Change-Id: Ie9aca05b88d5965ed1456396cc8ee763852fdee8
Merge "Disallow Heat-only functions in CloudFormation templates"