Use userdata instead of files for VM key-pair.

userdata script is supported by cloud-init package.
From now on Savanna expects images with cloud-init preinstalled.

Change-Id: Icfb34d79d22bfa5b8d682a10b42c81312ef5de8a
Fixes: bug #1193652
Merge "Use userdata instead of files for VM key-pair."