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.

I've also removed 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.