Add call to preview stack with template and params

This adds a call to preview what resources will be created with a given
template.  It is similar to a call to create a stack, but no resource
will actually be created and will just be returned for preview.  This
allows a user to know beforehand what will happen to their stacks once
they used such template.

The response is just the stack information along with a list of
resources that will be a part of that stack:

    {
      'stack': {
        // stack information similar to a call to stack_create/show
        'resources': [
          // resources similar to a call to resource_show
        ]
      }
    }

Because resources can be grouped in a StackResource (e.g. LoadBalancer,
ResourceGroup, InstanceGroup, etc), these will be inside their own list
inside the ``resources`` attribute:

    'resources': [r1, r2, [r3, r4, r5]]

Implements: blueprint preview-stack
Change-Id: I8b862110f00ac0cf31d96a86cbc134df895cc19b