Add dependencies and example sudoers file for image building

Posting for review, not merge (I'll do it in thirdparty).  @lzap, what do you think of the example sudoers file?  The idea is that you'd run:

    cp -a /etc/sudoers.d/foreman_discovery{.example,}
    sudo -u foreman RAILS_ENV=production ruby193-rake discovery:build_image
    rm -f /etc/sudoers.d/foreman_discovery

Bit nasty, but there's not much choice due to how it's implemented as a rake task.  Because it's using rake, it starts up the foreman application, which means it connects to the DB, which means for PostgreSQL + ident auth it has to be "foreman".