Reorg AutoScalingGroup code

The current class hierarchy of autoscaling groups in Heat is hindering
further improvements to both AWS version and OS version.  On one hand,
we don't want OS resources to extend AWS resources; on the other hand,
we have a lot proposals to enhance the OS version.  Also, the classes
should be split and relocated into different subdirectories.

Change-Id: Idedc141b9f63e4ed20b54bd297d2f737fe1249d1
Merge "Reorg AutoScalingGroup code"