Add SSH ProxyCommand functionality

Similar to vagrant's `ssh_info[:proxy_command]`, this adds a `:proxy_command` argument for SSH where an SSH proxy is required for a driver to communicate with remote instance(s).

I have tested (and am currently using) this with vagrant driver and vagrant-libvirt plugin where hypervisor is remote and instances live on hypervisor's internal private network.  Another (vagrant-specific) use-case is when `config.ssh.proxy_command` is placed in a user's Vagrantfile (or custom Vagrantfile ERB in kitchen-vagrant's case) to access remote instances when policy or network configuration requires it.