Expose cloudformation `UsePreviousTemplate` param

When provided, this parameter causes cloudformation to reuse the current
template, rather than a new one provided via TemplateBody or
TemplateURL.

Note that the CloudFormation API will explicitly reject requests containing
both `UsePreviousTemplate` and at least one of `TemplateBody` and
`TemplateURL` with the error `You cannot specify both
usePreviousTemplate and Template Body/Template URL`.

Please let me know your thoughts.

This commit fixes #786