vagrant: use "centos/7" box to support libvirt provider

The "centos/7" box is the official centos box and supports all the major
providers:

virtualbox Externally hosted (cloud.centos.org)
vmware_desktop Externally hosted (cloud.centos.org)
libvirt Externally hosted (cloud.centos.org)
hyperv Externally hosted (cloud.centos.org)

Where  bento/centos-7.3 only supports:

parallels Hosted by Vagrant Cloud (570 MB)
virtualbox Hosted by Vagrant Cloud (525 MB)
vmware_desktop Hosted by Vagrant Cloud (608 MB)

Signed-off-by: Sébastien Han <seb@redhat.com>