Adding sshkey functionality

cide now has an sshkey option that can be passed in as a CLI argument, or put in the .cide.yml config file.
This is a path to a private sshkey that exists on the system. This file will be copied into a temp file within the directory, then placed in the .ssh folder of the cide user and given correct permissions. cide will also create an sshconfig file that currently just has `StrictHostKeyChecking no` to stop ssh connections prompting to add hosts to the known hosts file
naming: ssh_key