Enhance to support expanding all virtual attributes

[abellotti]

There's an RFE for the REST API to expand all virtual attributes of a resource as per https://www.pivotaltracker.com/story/show/121850913
while one can ask for it specifically via something like expand=virtual_attributes, the API Client may want to do this behind the scene for the user (performance-reasonable/allowing) when specific resources are queried, Gives more flexibility on what ruby client code can do with the resource objects, i.e. if vm. expression, do something.