Vagrantfile: fallback on 'varant_variables.yml.sample'
Vagrantfile: fallback on 'varant_variables.yml.sample'

When using a vagrant command from the root directory of the repo, it
throws an error if no 'vagrant_variables.yml' file is present.

```
Message: Errno::ENOENT: No such file or directory @ rb_sysopen - /home/guits/workspaces/ceph-ansible/vagrant_variables.yml
```

Signed-off-by: Guillaume Abrioux <30a9df5a0293a6108675377fa1673f7aa713f915@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible