Enable use of aperture in ansible-playbook image

We have some magic configuration floating around which people use to enable use of the aperture jump host. Bring that functionality into this image:

• If APERTURE_USE_JUMP_HOST is set to a non-empty value, update ~/.ssh/config with the appropriate configuration.
• If APERTURE_USE_JUMP_HOST is set to a non-empty value make sure that APERTURE_USER is also set and interpolate it into the ~/.ssh/config.

Update README.md noting the two environment variables.