remove attributes from OS::Heat::AutoScalingGroup

The InstanceGroup attribute was inherited from the InstanceGroup
resource, but not implemented properly by OS::Heat::AutoScalingGroup. This
patch makes OS::Heat::AutoScalingGroup explicitly declare no attributes.

Change-Id: Iacf90b4590551a257ddd5442f28872e712da6c88
Closes-Bug: #1291789
Merge "remove attributes from OS::Heat::AutoScalingGroup"