Update `set_template_description()` to use `set_description()` internally

[scrthq]

Troposphere updated the underlying method name to be set_description() instead of add_description(). Without patching this behavior, setting a description on the top-level config file is unusable with the latest version of Troposphere, returning the following: AttributeError: 'Template' object has no attribute 'add_description'

[kukhmaz]

Any update on this fix?