coreos-install: Support http(s) URL for -c argument

* Pretty basic here, rather than requiring everyone to play tempfile
  games, just make the script handle a URL by fetching a cloud-config with
  curl.

* Shift the general tempdir and trap handling upwards such that the curl
  call can directly make use of the temp directory. Augment the trap
  sigspec to include all the usual suspects.

* Remove the final rm -rf and trap as the script is 1 line away
  from exiting at that point - and the same exact things specified in the
  EXIT trap will happen once the script exits after the final echo.