ncm-opennebula: add support for OpenNebula

Mmmm.... less spaghetti
```perl
if (!$secret) {
    $self->verbose("Output: $cmd_output") if $cmd_output;
    if ($?) {
       $self->error("Command failed: $cmd_err");
    } else {
       $self->verbose("Stderr: $cmd_err");
    }
}
```