Heat Stack update view/form

2 Part view for updating Heat Templates. The first page allows you to
select a new template for your stack. The second allows you to set new
template parameters for your stack. Like the launch stack workflow, this
is not a horizon workflow, but two separate forms.

Implements: blueprint heat-stack-update

Change-Id: I2854e9e4bb578be5187ef962808b93f11ac6b1f1