Add Ubuntu 18.04 to packaging tests (elastic#34139)

We intend to start supporting Ubuntu 18.04 with the 6.5.0 release. This
commit adds Ubuntu 18.04 to the packaging tests.

TESTING.asciidoc

@@ -402,6 +402,7 @@ These are the linux flavors supported, all of which we provide images for
 
 * ubuntu-1404 aka trusty
 * ubuntu-1604 aka xenial
+* ubuntu-1804 aka bionic beaver
 * debian-8 aka jessie
 * debian-9 aka stretch, the current debian stable distribution
 * centos-6

Vagrantfile

@@ -61,6 +61,15 @@ Vagrant.configure(2) do |config|
       SHELL
     end
   end
+  'ubuntu-1804'.tap do |box|
+    config.vm.define box, define_opts do |config|
+      config.vm.box = 'elastic/ubuntu-18.04-x86_64'
+      deb_common config, box, extra: <<-SHELL
+       # Install Jayatana so we can work around it being present.
+       [ -f /usr/share/java/jayatanaag.jar ] || install jayatana
+      SHELL
+    end
+  end
   # Wheezy's backports don't contain Openjdk 8 and the backflips
   # required to get the sun jdk on there just aren't worth it. We have
   # jessie and stretch for testing debian and it works fine.

buildSrc/src/main/groovy/org/elasticsearch/gradle/vagrant/VagrantTestPlugin.groovy

@@ -31,7 +31,8 @@ class VagrantTestPlugin implements Plugin<Project> {
             'opensuse-42',
             'sles-12',
             'ubuntu-1404',
-            'ubuntu-1604'
+            'ubuntu-1604',
+            'ubuntu-1804'
     ])
 
     /** All Windows boxes that we test, which may or may not be supplied **/