Commenting too late since this is merged, but it seems quite ugly to me to use `grep` when native Ruby is perfectly capable of the same thing, especially considering:

1. We're already doing pattern matching on `stdout` below.
2. A failure of the `grep` command is programmatically indistinguishable from a failure of `drbd-overview`.  Ideally this should raise an exception if `drbd-overview` fails, but a failure of the `grep` should be handled in a different way.