The local `~/.ssh/id_rsa.pub` shouldn't be required

[jschementi]

See https://github.com/jschementi/riker/blob/master/riker/api.py#L480

Currently, the ~/.ssh/id_rsa.pub from the machine running riker is used to push code to instances. This means that only a machine with that exact ~/.ssh/id_rsa and ~/.ssh/id_rsa.pub can push code, which makes it non-trivial to push code from another machine. Also, the id_rsa.pub is added to the base image, so that image needs to be recreated too.

Instead, either the instance's .pem should be used, or another one generated by riker (and stored in ~/.riker).