Need a way to extract lists efficiently using --key

[Clint Byrum]

There will be situations where we want to iterate over a list from Metadata in a script. So something like

for host in $(os-apply-config --key hosts --list --type netaddr) ; do
  do_logic_with $host
done

Note the addition of the fictional '--list'. I'm thinking it would print each item in the list with a newline after for easy consumption in shell.