fixes #8702 - adding ssh key pair integration for digital ocean

please do mind that this requires the user to put their SSH public keys into the machine using their config management tool after booting the machine, just like in EC2, otherwise, there won't really be any access to the machine (unless you get the private key from the DB)

Also, and this is critical - the configuration management should remove that key from the authorized_keys for root! the private key is stored as cleartext on the DB, and should only be used to boot the machine and start up the config management, be it puppet, chef or whatever else the user chooses to use. (that's a general comment, correct for DO, EC2 and OpenStack)