Support passing in raw ssh key

[nimajalali]

I'm attempting to use the Triton driver with Rancher.

Rancher runs the docker-machine command on the server side, this requires uploading the Triton ssh key to the server before hand and referencing the key path in the Rancher UI.

I'm looking to add support for passing in the raw ssh key so adding a machine can happen all within the Rancher UI.

Before I started the changes I wanted to get opinions on the approach.

We could create a new flag, --triton-key-material
or
We could overload the current flag --triton-key-path

By overload I mean we could try to interpret the triton-key-path as a file path first, if that fails we could assume the contents is a raw ssh key.

Thoughts?

[nimajalali]

So I've been looking into this more and realized that the Docker Machine Driver Interface requires the following function GetSSHKeyPath() string

This means the SSH key will have to be on disk. I thought about writing a temp file with the key data but once the machine is created docker machine expects the key to persist.

An alternative approach could be to separate the key used to authenticate with the Triton Cloud API and the key used to ssh into the docker host. Then we could allow an option to generate a key for the new host or use one on disk.

@fayazg Let's discuss offline...

[jwreagor]

You may want to revisit this once #6 (PR #10) is merged as things have changed. Not sure if better, worse, or the same but worth a look none the less.