Add heat-templates to pull-tools

The heat-templates repo has a set of elements
in hot/software-config/elements that are very
useful for alternative software-config hooks.

Some of us are starting to make use of these
elements on a daily basis so it would be nice to have
them automatically updated via pull-tools.

Change-Id: I38a304242f57bf37946b6a4fa009ffd47466182f
Merge "Add heat-templates to pull-tools"