Add local template generation tox task

Add local template generation tox task

This patch adds a local version of our template processing
routine so that developers can more quickly view the templates
that are actually getting generated. I've noticed multiple developers
now do a full deployment with 'overcloud deploy' only to download
the swift container with the generated templates. This simple task
avoids that step by allowing developers to generate it locally.

It also aims to preserve the ability to use t-h-t templates directly
with Heat (instead of going through Mistral) should users wish to do that.
The new undercloud heat installer requires the ability to generate
templates without requiring Mistral and Swift to do so.

Ideally the Mistral API workflow would use this same code
so perhaps in the future we might modify that routine to:

 -download swift tarball containing the templates
 -run this local routine that lives in t-h-t
 -re-upload the tarball of templates to the swift container

Change-Id: Ie664c9c5f455b7320a58a26f35bc403355408d9b