Allow empty 'config' for software deployment resources.
The use case would be for custom agents where the config
is already baked into the image, so these agents would
still want to:
read input_values to feed into the baked-in config
signal heat that deployment is complete, including output values
Change-Id: I87807065bf5821bcff23810d7e58c34eaa991f19
Closes-Bug: #1407387