A more reliable way to detect Ubuntu

lsb_release command is included in lsb-release package. Sometimes this is
not installed; ex: Docker official Ubuntu image.
/etc/os-release is included in base-files package,
so this is more reliable than lsb_release.

plugins/guests/ubuntu/guest.rb

@@ -2,7 +2,7 @@ module VagrantPlugins
   module GuestUbuntu
     class Guest < Vagrant.plugin("2", :guest)
       def detect?(machine)
-        machine.communicate.test("[ -x /usr/bin/lsb_release ] && /usr/bin/lsb_release -i 2>/dev/null | grep Ubuntu")
+        machine.communicate.test("test -r /etc/os-release && . /etc/os-release && test xubuntu = x$ID")
       end
     end
   end