Fix #1754 cancel_spot_instance_requests return val
Merge pull request #1811 from mbarrien/spotinstance

EC2 cancel_spot_instance_requests now returns a list of SpotInstanceRequest objects. Fixes #1811, #1754.
Altered Route53 to retry some 400 errors.
Fixes #1618.
Merge branch 'route53-retry-400s' into develop
Fixes #1618.