MINOR: Tag AWS instances with Jenkins build url (#4657)

This will allow us to trace leaked instances back to the job,
so that we can figure out what happened and fix the leak.

Reviewers: Ismael Juma <ismael@juma.me.uk>

Vagrantfile

@@ -141,7 +141,10 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
   def name_node(node, name, ec2_instance_name_prefix)
     node.vm.hostname = name
     node.vm.provider :aws do |aws|
-      aws.tags = { 'Name' => ec2_instance_name_prefix + "-" + Socket.gethostname + "-" + name }
+      aws.tags = {
+        'Name' => ec2_instance_name_prefix + "-" + Socket.gethostname + "-" + name,
+        'JenkinsBuildUrl' => ENV['BUILD_URL']
+      }
     end
   end