Add small reminder to use latest vagrant

Vagrant is fast moving tech and we've seen some bugs around users using
old distro packaged versions
(https://bugs.launchpad.net/kolla/+bug/1505045).

Change-Id: I37ae4452fd95e8b58aec228d89bd98214ac012df
backport: liberty

doc/devenv-vagrant.rst

@@ -40,6 +40,9 @@ On Fedora 22 it is as easy as::
 
     sudo dnf install vagrant ruby-devel libvirt-devel
 
+**Note:** Many distros ship outdated versions of Vagrant by default. When in
+doubt, always install the latest from the downloads page above.
+
 Next install the hostmanager plugin so all hosts are recorded in /etc/hosts
 (inside each vm)::