[digitalocean|compute] allow bootstrapping with keys rather than paths

* Allows specifying public_key and private_key rather than public_key_path and private_key_path. Useful for Heroku where you don't have direct access to the filesystem and need to specify keys in environment variables.
[digitalocean|compute] sync with latest master for new fog_test_server_attributes
[digitalocean|compute] fix merge conflict in fog_test_server_attributes
Merge pull request #2517 from 12spokes/digitalocean-deploy-keys

[digitalocean|compute] Allow bootstrapping with keys rather than paths