Add ANSIBLE option to Debian+Ubuntu build scripts

Ansible clients require the `python-apt` package to be installed in order to communicate with the host.  Eventually in future versions Ansible [will auto-install](https://github.com/ansible/ansible/issues/4079#issuecomment-24175465) this package if it is not found, but until then it is a hard requirement for it to be installed on the client.

Unfortunately I wasn't able to test the attached commit wrote properly; whenever I provide any environment variables (e.g. `ANSIBLE=1`, `PUPPET=1`, `CHEF=1`, etc.) to `build-ubuntu-box.sh` I never see the expected output like `installing puppet`.  Instead I temporarily added `python-apt` to the [`PACKAGES` variable](https://github.com/abevoelker/vagrant-lxc/blob/abf922f534fad15efbad677fd50deccb3d358b43/boxes/build-ubuntu-box.sh#L95) in `build-ubuntu-box.sh` to build the box I needed.  I'll open an issue for this in case someone wants to investigate, but I assume it's something I did wrong or something wrong with my specific environment.

I'm hoping that someone that can currently run the build scripts can do a smoke test that my trivial changes work.  Sorry again that it's not tested.