Fix service startup with acceptance tests (elastic#15900)

This commit fixes the startup of the Logstash service during packaging
tests by adding a minimal pipeline config. Without it, the service was
flapping from start to start and vice versa causing test flakiness.

Relates elastic#15784

qa/acceptance/spec/shared_examples/installed.rb

@@ -23,6 +23,7 @@
   before(:each) do
     logstash.uninstall
     logstash.install({:version => LOGSTASH_VERSION})
+    logstash.write_default_pipeline
   end
 
   it "is installed on [#{logstash.human_name}]" do

qa/acceptance/spec/shared_examples/installed_with_jdk.rb

@@ -28,6 +28,7 @@
   before(:each) do
     logstash.uninstall
     logstash.install({:bundled_jdk => true, :version => LOGSTASH_VERSION})
+    logstash.write_default_pipeline
   end
 
   after(:each) do

qa/rspec/commands.rb

@@ -135,6 +135,11 @@ def install(options = {})
       client.install(package)
     end
 
+    def write_default_pipeline()
+      # defines a minimal pipeline so that the service is able to start
+      client.write_pipeline("input { heartbeat {} } output { null {} }")
+    end
+
     def uninstall
       client.uninstall(name)
     end

qa/rspec/commands/base.rb

@@ -123,6 +123,10 @@ def download(from, to)
       run_command("curl -fsSL --retry 5 --retry-delay 5 #{from} -o #{to}")
     end
 
+    def write_pipeline(pipeline_string)
+      run_command("bash -c \"echo '#{pipeline_string}' >/etc/logstash/conf.d/pipeline.conf\"")
+    end
+
     def delete_file(path)
       run_command("rm -rf #{path}")
     end