Make some data available in server#show directly available

[tokengeek]

Issue in knife-brightbox rubiojr/knife-brightbox#21

This could be solved by having a cleaning interface on Server that respects
Demeter's law and avoids the chaining.

server.image.name was used but when an image has been later set to
"private", server#image returns nil.

If server#image_name was available based on the data already in the original
API call, a second API call would not be needed.

Need to ensure that these also appear in the collection API so they consistently
work in both cases.