provisioners/ansible: refer to directory that contains the generated inventory file

By referring to the directory that contains the generated inventory file,
users can easily provide more settings with additional files stored in
the same directory.

Related issues and comments:
- https://github.com/mitchellh/vagrant/pull/2991#issuecomment-35978587
- #3004
- #3434

/cc: @retr0h, @zeroem