Allow the setting of mode, group and owner on share directory.

Samba requires that the share directory has proper unix permissions before users can modify it. This pull request adds the functionality to set the owner, group and mode within the shares data bag.
After rereading the template, I see why, but I'd rather not break compatibility. What about putting the owner/group/mode data at the same level as `export`?

```json
{"shares": {
  "owner": "root",
  "group": "samba-users-who-write-samba-shares",
  "mode": "0775",
  "export": { }
}}