vagrant: use generic/debian9 box, fixes #7579

the debian/stretch64 was broken and never fixed.

Vagrantfile

@@ -427,7 +427,7 @@ Vagrant.configure(2) do |config|
   end
 
   config.vm.define "stretch64" do |b|
-    b.vm.box = "debian/stretch64"
+    b.vm.box = "generic/debian9"
     b.vm.provider :virtualbox do |v|
       v.memory = 1024 + $wmem
     end