Rename rackspace server ImageName, Flavor, UserData.

Justifications for each rename are as follows:
- ImageName -> image: the Nova API uses imageRef, however heat does
  a lookup from image name to id, so 'image' covers either name or id.
- Flavor -> flavor: the Nova API uses flavorRef, but in the future heat
  should be able to do a lookup on the flavor name as well, so calling the
  attribute 'flavor' will API future-proof that change.
- UserData -> user_data: maps directly to Nova API

Change-Id: I1327f7b33d744908714dfb7a4caeb6ec306196e7
Merge "Rename rackspace server ImageName, Flavor, UserData."