Allow DesiredCapacity to be zero

When updating or validating a AutoScaleGroup,
'DesiredCapacity' is ignored if the value is 0.

- Add validation for 'DesiredCapacity' if the value is 0,
- Don't ignore zero when updating.

Change-Id: I830aaf97ebed6b2c77c72b18dc8d222ce23f9a28
Partial-Bug: #1304423
Merge "Allow DesiredCapacity to be zero"