- Change ImportOSIntoInstance not to get any "os_disk" and "swap_disk"
  arguments but to accept multiple target images to import, and to
  return a list of booleans with the result of each import
- Change the relevant rpc call and the only caller to conform
- Pass arguments to the import script through the environment
- Run one import os script for each disk image, passing an IMPORT_DEVICE

Reviewed-by: iustinp