bash completion for subcommands

Couple of things:
- I'd love for there to be a better place to put _chef-server-ctl.sh, but boring-old-bash doesn't autoload from a completion-specific dir, so I used /etc/profile.d (which we already use for chef.sh)
- I'd love to have completion for the arguments/flags of subcommands, but they aren't exposed in omnibus-ctl and I'm avoiding chef-server-ctl --help for speed's sake