changing username from "ubuntu" to the correct one "vagrant" for ubuntu

Vagrantfile

@@ -13,7 +13,7 @@ SUPPORTED_OS = {
   "coreos-stable" => {box: "coreos-stable",      bootstrap_os: "coreos", user: "core", box_url: COREOS_URL_TEMPLATE % ["stable"]},
   "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: "ubuntu"},
+  "ubuntu"        => {box: "bento/ubuntu-16.04", bootstrap_os: "ubuntu", user: "vagrant"},
 }
 
 # Defaults for config options defined in CONFIG