Include file content in scenario tests

Use the heatclient utilities for loading referenced files and passing them
in the files dict when running scenario tests. This allows testing nested
stack scenarios in a way that is representative of how users would
encounter them.

Change-Id: I6693a0a9e0e1af42b69ade12c8da6f12d370eb49
Story: #1739447
Task: 22218
2 files changed