Allow simpler scale exprs in transformer.conversions

For convenience of deployers, allow scale expression to be
expressed as for example:

  100.0 / (10**9 * (resource_metadata.cpu_number or 1))

instead of requiring that the full dict lookup syntax with
default value is used:

  100.0 / (10**9 * (resource_metadata.get('cpu_number', 1))

Unknown items evaluate to false in boolean expressions such
as:

  resource_metadata.non.existent or 1.0

Change-Id: I2f1f84641222057b4d6213dcfd1c16f23ae9f273
Merge "Allow simpler scale exprs in transformer.conversions"