Pass ``--os`` during nightly install.

We need to pass ``--os`` to the nightly installer in order to avoid some
of the issues we have seen while installing it on RHEL 7. According to
the installer:

```bash
Usage: ./setup.rb [options]
        --os [OS]                    Set OS manually
        --devel                      Setup a development environment
        --devel-user [USERNAME]      User to setup development environment for
        --installer-options [OPTIONS]
                                     Options to pass to katello-installer
        --skip-installer             Skip the final installer command and print instead
        --deployment-dir [DIRECTORY] Set a custom path for installing to (defaults to /home/USERNAME)
        --version [VERSION]          Set the version of Katello to install nightly|2.0
```
Merge pull request #62 from omaciel/pass-os-flag

Pass ``--os`` during nightly install.