Batch mode

# Background

User may want to concurrently run kdebug on multiple machines to get an aggregated result.

# Design

## Machine discovery

### Static discovery

User explicitly provides a list of machines:

```
kdebug --batch.machines machine1,machine2
```

### Kubernetes node discovery

Automatically discover machines that are registered in Kubernetes via API:

```
kdebug --batch.kubemachines
```

Also we can support some basic filtering:

```
kdebug --batch.kubemachines --batch.kubemachines.ready=False
```

## Execution

### SSH executor

SSH executor assumes user already has SSH access to machines. The workflow looks like:

1. SCP kdebug binary to `/tmp/kdebug`.
2. Remote execute `kdebug` while specifying output in JSON format.
3. Parse output and deserialize results.