`triton profile` should generate separate keys for Docker

[arekinath]

Currently triton profile setup subcommands like docker-setup try to save a decrypted copy of your private key to disk for use with Docker. Since DOCKER-1005 has been rolled out very widely at this point, we can change this safely to instead generate a separate key + certificate for Docker that's signed by a main account key. This will be a safer setup from a security perspective and also a nicer user experience for people using an SSH agent with triton.

[arekinath]

Initial CR: https://cr.joyent.us/#/c/3860/

[bahamat]

This was implemented in 5734123.