Fix the way self.properties is checked for a value.

Fix the way self.properties is checked for a value.

The following only checks if the key is in the properties, and
all schema keys are, so it is not the way to check if a value
has been set.

if 'DesiredCapacity' in self.properties:
change to:
if self.properties:

Fixes: #199
Change-Id: I6bcb3e74420031532dc249aafe85d5a428d0a80e
Signed-off-by: Angus Salkeld <asalkeld@redhat.com>