communicator/ssh, builder/digitalocean: fix new SSH API from upstream

Upstream SSH API remove the key type specific New Public Key functions, in favor of a generic function. This needs further testing, as I don't have a good key based authentication test case, but based on what I see in the upstream code this should resolve the issues.

Error messages resulting from these API changes are showing up in the Travis CI logs.

/cc @kelseyhightower