Add root_block_device variables

### What?
Add the following variables with this expected default values
- root_block_device_volume_type = "gp2"
- root_block_device_volume_size = 20
- root_block_device_encrypted   = true

```
  root_block_device = [
    {
      volume_type = "gp2"
      volume_size = 20
      encrypted   = true
    },
  ]
```

### Why?
To grant this expected values by default.  