autoscale scaling policy desiredCapacity support

Add a "desired_capacity" parameter to all of the ScalingPolicy
manipulation functions, and use it to set the "desiredCapacity"
attribute on the json object for created and updated policies.
update the add_policy methods too.
update tests for desired_capacity support.
Merge pull request #208 from radeex/desired-capacity

Support for the "desiredCapacity" parameter for policies