Do not use the '+' operation with dict_items()

This operation is undefined in Python3, list() must be called first.

Change-Id: Ia8e06d5be6a0bd4426ad5ea5028fbe7d85fa12e2

heatclient/v1/shell.py

@@ -205,7 +205,7 @@ def do_stack_preview(hc, args):
         'disable_rollback': not(args.enable_rollback),
         'parameters': utils.format_parameters(args.parameters),
         'template': template,
-        'files': dict(tpl_files.items() + env_files.items()),
+        'files': dict(list(tpl_files.items()) + list(env_files.items())),
         'environment': env
     }