AWS::NoValue

In CFN, there is now a psudo param that allows the key/value pair to be completely removed if assigned the value of the psudoparam. For example:
foo: {"Ref" : "AWS::NoValue"}

causes foo to be removed from the resource rather then having a None value. This is best used in combination with Fn::If

This feature would allow paramaters to be truely optional rather then needing multiple template versions, one that allows the value to be set, and another that doesn't have the key mentioned at all. For example, set a security group on a neutron port if specified, and use the neutron default if not set at all by removing the security_group key.