vagrant: Default box to centos/7
vagrant: Default box to centos/7

We don't use ceph/ubuntu-xenial anymore but only centos/7 and
centos/atomic-host.
Changing the default to centos/7.

Resolves: #4036

Signed-off-by: Dimitri Savineau <a50a620e679eed14f4e99998b821cea8884cf228@redhat.com>

ceph/ceph-ansible,ceph/ceph-ansible