Fix parameter handling for boto_rds with boto3
Fix parameter handling for boto_rds with boto3

The way that optional parameters are handled forces some of them to be
defined even when that is not necessary. Updated handling to use a
mapping of snake_case to CamelCase names and only include defined
parameters in the call to boto for the `create` and `modify` actions.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt