Pass before and after snippets to template diff

The before template should be passed explicitly, rather than relying on
some state stored in the object.

Change-Id: I5b0af8da2e1df3f6ad3291554cbbc243b51e3fe5
Changes for HOT hello world template processing

This patch contains the base enabling code required for processing a most
simple HOT template ("hot hello world"), so that the template can be
parsed and a stack with a single instance can be deployed.

Contributes to blueprint hot-hello-world

Change-Id: I4f0a04a06fab2f5472cb4481dcdf10d5239d5def
Merge "Changes for HOT hello world template processing"
Merge "Pass before and after snippets to template diff"
Merge branch 'master' into bp-dev/rackspace-cloud-servers-provider

Conflicts:
	heat/engine/resource.py

Change-Id: Ifffe9536bfd01f27e3c8a2b219b3f32a2baa5f3f