Update nested stacks in parallel

This parallelises updates for AWS::CloudFormation::Stack resources, but not
for autoscaling groups. Other types of nested stacks (the old LoadBalancer
and DBInstance resources, and provider templates) have not yet implemented
stack updates.

Change-Id: I86e1d5289487b34712c248bc5e68e62cc4651ec6
Merge "Update nested stacks in parallel"