guests/ubuntu: Revert detection

- Semi-reverts GH-7524
- Fixes GH-7625

plugins/guests/ubuntu/guest.rb

@@ -2,7 +2,22 @@ module VagrantPlugins
   module GuestUbuntu
     class Guest < Vagrant.plugin("2", :guest)
       def detect?(machine)
-        machine.communicate.test("test -r /etc/os-release && . /etc/os-release && test xubuntu = x$ID")
+        # This command detects if we are running on Ubuntu. /etc/os-release is
+        # available on modern Ubuntu versions, but does not exist on 14.04 and
+        # previous versions, so we fall back to lsb_release.
+        #
+        #   GH-7524
+        #   GH-7625
+        #
+        machine.communicate.test <<-EOH.gsub(/^ {10}/, "")
+          if test -r /etc/os-release; then
+            source /etc/os-release && test xubuntu = x$ID
+          elif test -x /usr/bin/lsb_release; then
+            /usr/bin/lsb_release -i 2>/dev/null | grep -q Ubuntu
+          else
+            exit 1
+          fi
+        EOH
       end
     end
   end