So we also need to allow them to override the bosh volume size? Add another override env variable:

``` ruby
# the default size of a microbosh volume in Gbs
def default_microbosh_volume_size
  $['BOSH_VOLUME_SIZE'] || $['MICROBOSH_VOLUME_SIZE'] || $['MICRO_BOSH_VOLUME_SIZE'] || 16
end
```

And when we set `settings[:bosh][:persistent_disk]` use that value and multiply it by 1024.