Document autoscaling resource properties

Adds descriptions to the properties of:
* AWS::AutoScaling::LaunchConfiguration
* AWS::AutoScaling::AutoScalingGroup
* AWS::AutoScaling::ScalingPolicy
* OS::Heat::InstanceGroup
and wraps attribute descriptions with gettext _(

Change-Id: Ie825c2b122a6c2ce8ff83f9beb9f98487aaa8d54
Closes-Bug: #1224131
Merge "Document autoscaling resource properties"