add a more robust check for coreos
add a more robust check for coreos

bryanl/doit,bryanl/doit,bryanl/docli,bryanl/docli,xmudrii/doctl,bryanl/doit,digitalocean/doctl,bryanl/docli,thebyrd/doit,xmudrii/doctl,digitalocean/doctl,bryanl/doit