os/booting-on-digitalocean: Warn about "private" networking
os/booting-on-digitalocean: Warn about "private" networking

Adopt @robszumski's version of changes suggested by @kevinsimper
to warn digital ocean users about implications of "private"
networking in the DO context.

Supersedes #724. Fixes #718.

joshix/docs,atxwebs/docs,jonboulle/docs,endocode/coreos-docs,coreos/docs