Run packaging tests on RHEL 8 (#41662)

TESTING.asciidoc

@@ -374,6 +374,7 @@ These are the linux flavors supported, all of which we provide images for
 * debian-9 aka stretch, the current debian stable distribution
 * centos-6
 * centos-7
+* rhel-8
 * fedora-28
 * fedora-29
 * oel-6 aka Oracle Enterprise Linux 6

Vagrantfile

@@ -127,6 +127,12 @@ Vagrant.configure(2) do |config|
       sles_common config, box
     end
   end
+  'rhel-8'.tap do |box|
+    config.vm.define box, define_opts do |config|
+      config.vm.box = 'elastic/rhel-8-x86_64'
+      rpm_common config, box
+    end
+  end
 
   windows_2012r2_box = ENV['VAGRANT_WINDOWS_2012R2_BOX']
   if windows_2012r2_box && windows_2012r2_box.empty? == false

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

@@ -30,6 +30,7 @@ class VagrantTestPlugin implements Plugin<Project> {
             'oel-6',
             'oel-7',
             'opensuse-42',
+            'rhel-8',
             'sles-12',
             'ubuntu-1604',
             'ubuntu-1804'