Further split ASG code.

This patch splits AWS and Heat version of AutoScalingGroups into
separate files and renames the autoscaling.py to instance_group.py.

The ugly class hierarchy will be fixed in following patches.

Change-Id: I4b776e3f4b74df3e083fcc6f8ba583822ca16580
Implements: partial-blueprint reorg-asg-code
Merge "Further split ASG code."