Minor corrections for correcting some inconsistencies in ec2.json

Hi,
  I'm trying to write an SDK based on definitions from the JSON files. (See https://github.com/pplu/aws-sdk-perl).

  I've detected some inconsistencies that I'm pushing back:
   -  Renamed LaunchSpecification for SpotInstances call: There is another LaunchSpecification shape with the same name, but different definition
   - Monitoring attribute for SpotInstances is a structure, but like all structures, didn't have a shape_name
   - Reserved Instances ProductDescription Type was being used for Spot Instances, but they are different
   - Added m3 medium and large instance types.

Best Regards,

Jose Luis