Lower Heat max_template_size for Tempest

The default size leaves a very large padding for users to create larger
templates, but for testing and development a 10kB template is plenty.

This value is specifically meant to mirror upcoming changes to tempest
so that they are unified and can be tested in lock-step.

Change-Id: I0ea9798018a6d864ac04429c3ac89fb374583fb6
Merge "Lower Heat max_template_size for Tempest"