Allow strings in heat SW config configSection

For non-structured configs, configSection cannot be restricted to a
Map. Patch removes the contract type, although it is worth revisiting
this to refactor slightly in future (which should be possible without
changing the interface).

Change-Id: I179e350a724965c97e90c3a1927f955c20cc2a72
Closes-Bug: 1358046
Merge "Allow strings in heat SW config configSection"