"bosh ssh" prompts for which instance to ssh

Usage:

    $ bosh ssh
    1. service_gateways/0
    2. postgresql_service_node/0
    3. redis_service_node/0
    Choose an instance: 2
    Enter password (use it to sudo on remote host): *
    Target deployment is `cf-services-dfw2-prod-alt'

    Setting up ssh artifacts

    Director task 7791

    Task 7791 done
    Starting interactive shell on job postgresql_service_node/0
Update for style and clarity of 'pr/390'