Only change owner of automatically created directories

Make sure to not change the owner and group of existing destinations in
the method inject_directory.
What about replacing lines 155-160 with

```ruby
chown_cmd = " && chown #{owner_group} '#{destination}'" if owner_group
mkdir_cmd = "test -d '#{destination}' || (mkdir -p '#{destination}' #{chown_cmd} )"
```
? That's a bit more readable imo.