PublicKey SSH Login with kitchen login

- Configuration directory is created similar to .kitchen/kitchen-docker/instance-platform/
- Public key pair is generated (In the configuration directory) on the HOST machine and the public key is uploaded as the 'authorized_keys' file in the kitchen user's .ssh directory. 
- Appropriate permissions are also set on the .ssh directory and the authorized keys file.
- DockerFile is generated and saved to the configuration directory (It is then used from there by passing the file path to the docker command).

These changes allow passwordless login to created instances using the 'kitchen login' command.