Add 'quiet' option to list-units cmd

[vnugent]

Please consider adding --quiet option to list-units command to show only gear names similar to -q option in 'docker ps'. It'd be useful to be able to stop and delete apps in batches
gear delete $(gear list-units --quiet)

[smarterclayton]

Fixed - -q (and also -a to list all containers that are stopped is being merged now). There's also gear purge which burns it all down.

[davidpelaez]

@vnugent you can now run gear list-units -a -q 2> /dev/null to obtain something like this:

app.4
busy.1
busy.2
sample2