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>

Vagrantfile

@@ -17,7 +17,7 @@ SUPPORTED_OS = {
   "coreos-alpha"  => {box: "coreos-alpha",       bootstrap_os: "coreos", user: "core", box_url: COREOS_URL_TEMPLATE % ["alpha"]},
   "coreos-beta"   => {box: "coreos-beta",        bootstrap_os: "coreos", user: "core", box_url: COREOS_URL_TEMPLATE % ["beta"]},
   "ubuntu"        => {box: "bento/ubuntu-16.04", bootstrap_os: "ubuntu", user: "vagrant"},
-  "centos"        => {box: "bento/centos-7.3",   bootstrap_os: "centos", user: "vagrant"},
+  "centos"        => {box: "centos/7",           bootstrap_os: "centos", user: "vagrant"},
 }
 
 # Defaults for config options defined in CONFIG