removed support for billingProducts in ec2.image and updated docs

* removed references to billingProducts in ec2.image and associated tests because despite the discussion in #1703, I cannot find references in any of the AWS EC2 wsdls supplied here (https://aws.amazon.com/releasenotes/Amazon-EC2) which contain a billingProducts element.
* removed references to instanceLifecycle in ec2.image for reasons similar to above
* added class variable documentation to ec2.image
* removed duplicate entry for groups in class variable documentation for ec2.instance