update :data_bag_secret_file to use config instead of constant

This PR is a porting of a change made to the LXC cookbook per LXC cookbook PR #35

https://github.com/hw-cookbooks/lxc/pull/35/files

Original problem without this PR was this during a 'vagabond init':

```
NameError
---------
uninitialized constant Chef::EncryptedDataBagItem::DEFAULT_SECRET_FILE
```